[FINAL] Adding an endpoint to fetch bitcoin block headers

[THLO]

The current Bitcoin integration API does not expose any information about block headers. This information would be useful to verify blocks (e.g., obtained using HTTPS outcalls) by comparing the Merkle root hash with the root hash in the block header.

This PR introduces a new endpoint bitcoin_get_block_headers.

The PR also reorganizes some Bitcoin integration related definitions to make things easier to read from top to bottom.