Broadcast a signed BTC staking transaction to the Bitcoin network.

This is step 6 (final step) of the BTC staking flow:

1. POST transactions/prepare/pre-delegate — Server prepares PSBTs and POP message
2. Client signs the PSBTs and POP message with their BTC key
3. POST transactions/prepare/delegate — Server assembles the MsgCreateBTCDelegation
4. POST transactions/broadcast/babylon — Client signs and broadcasts the Babylon transaction
5. GET stakes/:txId/status — Client polls until delegation reaches VERIFIED or ACTIVE state
6. POST transactions/broadcast/bitcoin — Client broadcasts the signed Bitcoin staking transaction (you are here)

Important: Only broadcast the Bitcoin staking transaction after the delegation has been verified in step 5. Broadcasting before verification may result in lost funds.