feat(rpc): finalize SyncChainMmr gRPC endpoint

[kkovaacs]

Based on the discussion in PR #2069 and issue #2044 we've converged on some smaller changes to the SyncChainMmr endpoint.

Changes made to the current interface:

• Removal of the arbitrary block number sync target. Clients can now choose which finality level they'd like to reach but always sync up to the chain tip (proven or committed).
• We now return the validator signature for the chain tip block header in the response.

The proposed protobuf interface (implemented by this PR) is the following:

// Finality level we'd like to sync up to.
enum FinalityLevel {
    FINALITY_LEVEL_UNSPECIFIED = 0;
    // Sync up to the latest committed block (chain tip).
    FINALITY_LEVEL_COMMITTED = 1;
    // Sync up to the latest proven block.
    FINALITY_LEVEL_PROVEN = 2;
}

// Chain MMR synchronization request.
message SyncChainMmrRequest {
    // Block number from which to synchronize (inclusive). Set this to the last block
    // already present in the caller's MMR so the delta begins at the next block.
    fixed32 current_block_height = 1;

    // Sync target: either the committed or the proven tip.
    FinalityLevel finality_level = 2;
}

// Represents the result of syncing chain MMR.
message SyncChainMmrResponse {
    // For which block range the MMR delta is returned.
    BlockRange block_range = 1;

    // Data needed to update the partial MMR from `request.block_range.current_block_height + 1` to
    // the sync target.
    primitives.MmrDelta mmr_delta = 2;

    // Block header for the sync target.
    blockchain.BlockHeader block_header = 3;

    // Validator signature for the sync target.
    blockchain.BlockSignature block_signature = 4;
}

Closes #2044

[bobbinth]

Looks good! Thank you! I left one small comment inline.

[bobbinth]

Are the two options still relevant? Seems like we always provide block_number now.