Prepare unsigned PSBTs and a Proof of Possession message for BTC staking on Babylon.

This is Exchange 1 of the BTC staking flow:

1. POST transactions/prepare/pre-delegate — Server creates staking TX, slashing PSBTs, unbonding PSBTs, and a 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

Inputs Required:

• Staker's BTC address and x-only public key
• Staking amount (satoshis), timelock (blocks), and fee rate (sat/vB)
• Staker's Babylon address (bbn prefix)

Returns:

• Slashing and unbonding slashing PSBTs to sign
• POP message and its expected signature type
• Staking PSBT to sign
• Session state to pass back in step 3
• bitcoinStakingTxId to poll delegation status in step 5