implement get watcthower_challenge_tx rpc endpoint

[ekrembal]

get_watcthower_challenge_tx(deposit_txid, operator_idx, seq_col_idx, kickoff_connector_idx)

• Check wheter the kickoff is malicious or not
• Check if the header chain collected enough total work
• Generate header chain proof with only total_work output
• Convert the proof to the groth16 proof
• Sign the proof with winternitz
• sign watcthower_challenge_tx
• Return raw tx