Sending BTC requires 2 rounds of communication between wallet and bittapd, because bittapd needs end users to sign only once.
Step1: TransferBtc
Wallet calls this to transfer BTC to a specific address. Psbt is returned for the end user's sign.
Step 2: PublishTransferBtc
The end user will sign the psbt returned from Step 1, and before call this to broadcast to BTC blockchain, the signed psbt needs to be finalized and extracted.
