Relayer: Potential Header type incompatibility

[olga24912]

In the contract, we use btc_type::Header, while in the relayer, bitcoincore_rpc::bitcoin::block::Header is used. It's not certain that they have the same serialization in JSON.

Moreover, the relayer uses the function get_block_header, which has now been replaced by get_last_block_header. And get_last_block_header returns an ExtendedHeader, not a Header.