Headers and Web fixes

Fixes:

• fix request headers handling (see #98)
• fix sequence number handling for web (see #97)

Breaking changes:

• The sequence number has been changed from int to BigInt

Null-Safety improvements

Null-Safety improvements:

With the introduction of null-safety in dart, the sdk was migrated automatically, which resulted in all class member variables becoming nullable. Most places have been adapted in the mealtime, but there are still a few places that need to be updated.
This release updates the remaining places, especially classes representing horizon responses but also different xdr classes.

Big Amounts on Web:

This release also adds support for big amounts on web for many operations such as, change_trust,
create_account, payment, path_payment_strict_send, path_payment_strict_receive, manage_sell_offer,
manage_buy_offer, create_passive_sell_offer, clawback, create_claimable_balance, liquidity_pool_deposit,
liquidity_pool_withdraw. See also: #96

Breaking changes:

ContractCreditedEffectResponse and ContractDebitedEffectResponse moved from liquidity_pools_effects_responses.dart to soroban_effects_responses.dart

LiquidityPoolDepositedEffectResponse: reservesDeposited is now a list of AssetAmount instead of ReserveResponse

LiquidityPoolWithdrewEffectResponse: reservesReceived is now a list of AssetAmount instead of ReserveResponse

LiquidityPoolRevokedEffectResponse: reservesRevoked is now a list of LiquidityPoolClaimableAssetAmount instead of ReserveResponse

TrustLineFlagsUpdatedEffectResponse moved from misc_effects_responses.dart to trustline_effects_responses.dart

CreatePassiveSellOfferOperationResponse: removed offerId since it is always null

LiquidityPoolDepositOperationResponse: reservesMax & reservesDeposited are now a list of AssetAmount instead of ReserveResponse

Operation: id is now a String instead of int

EffectsRequestBuilder forOperation(String operationId): operationIdis now aStringinstead ofint`

Future<OperationResponse> operation(String operationId): operationId is now a String instead of int

Soroban improvements

1. Soroban Server improvements:

• add getAccountand getContractData methods
• add missing fields oldestLedger and latestLedger to the get health response
• add useful xdr conversions: ledgerEntryData and keyVal for LedgerEntry
• add useful xdr conversions: xdrTransactionEnvelope, xdrTransactionResult, xdrTransactionMeta for GetTransactionResponse
• add missing field txHash to EventInfo
• add useful xdr conversion: valueXdr to EventInfo
• improve simulate transaction result: xdr and auth are not nullable any more

2. SEP-11 (TxRep):

• add soroban support

3. XDR helper methods

• multiple useful additional helper methods in the soroban and classic XDR classes.

4. Breaking changes:

• renamed Row to OrderBookRow for the order book response
• typo: renamed statusChangesto stateChanges for the soroban simulate transaction response

Protocol 21 Support

adds support for protocol 21. see #93

Update Soroban RPC Args

• update soroban RPC args for sendTransaction and getTransaction
• allow optional httpClient to init StellarSDK. See #91

SEP-08 support

adds support for SEP-08. See doc.

Update SEP-01 & improvements

• update SEP-01 currency fields: add contract field
• improve streams in the request builders. see: #90
• update the request builder to maintain the original path and query. see: #89

Update SEP-06

• update SEP-09 fields (see #86)
• update SEP-06 to reflect current SEP-06 doc version. See also: SDK doc. Resolves #87

SEP-38 Support

• add sep-38 support see doc

Sep-009 update

update and extend SEP-009 kyc fields