rpc: expose the exact data used by bridge nodes

[cmwaters]

Summary

This issue comes from a slack conversation. As I understand it, bridge nodes in celestia-node subscribe and listen for block events. For each new block, they then query the commit and validator set at the height of the block in order to verify it and process it accordingly (rebuild the extended square). Not only is redundant data being passed over the wire i.e. LastCommit, Evidence, ResponseBeginBlock and ResponseEndBlock, but the process requires two extra network round-trips and additional retry and error handling logic on the celestia-node side. It is trivial if instead, celestia-core exposes the correct data contained in a single request.

Proposal

Create a new event

type SignedDataEvent struct {
    header types.Header
    commit types.Commit
    vals types.ValidatorSet
    data types.Data
}

and RPC endpoint

func SignedData(ctx context.Context, height *int64) (*ctypes.ResultSignedData, error)

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