Implement Horizon Protocol 14 API changes

[tamirms]

This PR implements support for Horizon's Protocol 14 API changes. Here is a summary of all the API changes:

New objects

• Claimable Balance with the following fields:
  • id - balance ID,
  • paging_token - paging token,
  • asset - asset available to be claimed (in canonical form),
  • amount - amount available to be claimed (string, like all amounts),
  • sponsor - sponsoring account ID (can be null),
  • last_modified_ledger - sequence number of the ledger when the balance was last modified,
  • claimants - list of objects:
    • destination - destination account ID,
    • predicate - predicate required to claim a balance (see below).

Modified objects

• Account object:
  • New sponsor field (account ID, can be null).
  • New num_sponsoring field - number of reserves sponsored by this account,
  • New num_sponsored field - number of reserves sponsored for this account.
• New sponsor field (account ID) in Account > Balance object (only for non-native assets, can be null).
• New sponsor field (account ID) in Account > Signers object (can be null).
• New sponsor field (account ID) in Account's Data object (can be null).
• New sponsor field (account ID) in Offer object (can be null).

New endpoints

• /claimable_balances - the list of Claimable Balance objects with the following parameters (only one param per request allowed):
  • asset - find all claimable balances with the given asset (in canonical form),
  • claimant - find all claimable balances with the given claimant account ID,
  • sponsor - find all claimable balances sponsored by a given sponsor account ID.
• /claimable_balances/{id} - a single Claimable Balance object.

Modified endpoints

• /accounts can now by filtered by sponsor (new GET param).
• /offers can now by filtered by sponsor (new GET param).

New operations

• create_claimable_balance with the following fields:
  • asset - asset available to be claimed (in canonical form),
  • amount - amount available to be claimed,
  • claimants - list of claimants with predicates (see below):
    • destination - destination account ID,
    • predicate - predicate required to claim a balance (see below).
• claim_claimable_balance with the following fields:
  • balance_id - unique ID of balance to be claimed,
  • claimant - account ID of a claimant.
• begin_sponsoring_future_reserves with the following fields:
  • sponsored_id - account ID for which future reserves will be sponsored.
• end_sponsoring_future_reserves with the following fields:
  • begin_sponsor - account sponsoring reserves.
• revoke_sponsorship with the following fields:
  • account_id - if account sponsorship was revoked,
  • claimable_balance_id - if claimable balance sponsorship was revoked,
  • data_account_id - if account data sponsorship was revoked,
  • data_name - if account data sponsorship was revoked,
  • offer_id - if offer sponsorship was revoked,
  • trustline_account_id - if trustline sponsorship was revoked,
  • trustline_asset - if trustline sponsorship was revoked,
  • signer_account_id - if signer sponsorship was revoked,
  • signer_key - if signer sponsorship was revoked.

New effects

• claimable_balance_created with the following fields:
  • balance_id - unique ID of claimable balance,
  • asset - asset available to be claimed (in canonical form),
  • amount - amount available to be claimed.
• claimable_balance_claimant_created with the following fields:
  • balance_id - unique ID of a claimable balance,
  • asset - asset available to be claimed (in canonical form),
  • amount - amount available to be claimed,
  • predicate - predicate required to claim a balance (see below).
• claimable_balance_claimed with the following fields:
  • balance_id - unique ID of a claimable balance,
  • asset - asset available to be claimed (in canonical form),
  • amount - amount available to be claimed,
• account_sponsorship_created with the following fields:
  • sponsor - sponsor of an account.
• account_sponsorship_updated with the following fields:
  • new_sponsor - new sponsor of an account,
  • former_sponsor - former sponsor of an account.
• account_sponsorship_removed with the following fields:
  • former_sponsor - former sponsor of an account.
• trustline_sponsorship_created with the following fields:
  • sponsor - sponsor of a trustline.
• trustline_sponsorship_updated with the following fields:
  • new_sponsor - new sponsor of a trustline,
  • former_sponsor - former sponsor of a trustline.
• trustline_sponsorship_removed with the following fields:
  • former_sponsor - former sponsor of a trustline.
• claimable_balance_sponsorship_created with the following fields:
  • sponsor - sponsor of a claimable balance.
• claimable_balance_sponsorship_updated with the following fields:
  • new_sponsor - new sponsor of a claimable balance,
  • former_sponsor - former sponsor of a claimable balance.
• claimable_balance_sponsorship_removed with the following fields:
  • former_sponsor - former sponsor of a claimable balance.
• signer_sponsorship_created with the following fields:
  • signer - signer being sponsored.
  • sponsor - signer sponsor.
• signer_sponsorship_updated with the following fields:
  • signer - signer being sponsored.
  • former_sponsor - the former sponsor of the signer.
  • new_sponsor - the new sponsor of the signer.
• signer_sponsorship_removed with the following fields:
  • former_sponsor - former sponsor of a signer.

predicate field

predicate field is a JSON representation of xdr.ClaimPredicate as defined in CAP-23 and is a requirement that needs to be satisfied to claim the balance. It is a recursive structure that can be represented in JSON using for example the following Golang struct:

type claimPredicateJSON struct {
	And           *[]claimPredicateJSON `json:"and,omitempty"`
	Or            *[]claimPredicateJSON `json:"or,omitempty"`
	Not           *claimPredicateJSON   `json:"not,omitempty"`
	Unconditional bool                  `json:"unconditional,omitempty"`
	AbsBefore     *time.Time            `json:"absBefore,omitempty"`
	RelBefore     *int64                `json:"relBefore,omitempty"`
}

In the next I will add support for creating transactions with claimable balance and sponsorship operations. The final PR will update the changelog.