chainSpec: Add Checkpoint extension to the chainSpec

[lexnv]

The Checkpoint extension is needed for the lightclient to synchronize faster to the head of the chain.

This is a replacement for the current lightSyncState, which doesn't expose directly any particular data about the grandpa or babe; and therefore doesn't cause a breaking change when switching consensus mechanisms (sassafras).

Instead, the finalized block's :code section is exposed indirectly in a storage proof. Then, the user of the checkpoint will regenerate the state of the consensus using the runtime APIs.

The checkpoint is a Merkle proof that consists of:

• :code and :heappages storage proofs
• BabeApi_current_epoch, BabeApi_next_epoch, BabeApi_configuration, GrandpaApi_grandpa_authorities, and GrandpaApi_current_set_id call proofs

"checkpoint": {
    "header": "...",
    "callProof": "..."
}

For now, the checkpoint extension is added next to the lightSyncState. A future PR will deprecate and remove the lightSyncState.

This PR validates the proposal from: #60; as well as unblocks the chainSpec V2 method which awaits a consensus on the checkpoint format: paritytech/json-rpc-interface-spec#124.

Testing Done

• Checked that the storage proof contains the :code section needed for users to regenerate the consensus state.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: cargo-clippy
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5162372