Wrap BTC APIs to support wallet/signer interface

[ShookLyngs]

Feature request

Currently the BTC APIs are designed to return a PSBT, like the following exmaple:

// Build
const psbt = await sendBtc({
  from: account.address,
  to: {
    address: 'btc_to_address',
    value: 10000,
  },
  source,
});

// Sign
psbt.signAllInputs(account.keyPair);
psbt.finalizeAllInputs();

// Broadcast
const tx = psbt.extractTransaction();
const txid = await service.sendTransaction(tx.toHex());

@duanyytop wish the build/sign/send process can be wrapped/simplified to a single call, like:

const txid = await sendBtc({
  from: signer,
  to: {
    address: 'btc_to_address',
    value: 10000,
  },
  source,
  service,
});

Estimation

In my mind, it would take some steps to support this feature:

1. Keep the current design of the BTC APIs, but rename them to be more accurate (sendBtc -> buildSendBtcTx)
2. Design a Wallet interface that provides publicKey, address and signTransaction to the APIs
3. Wrap BTC APIs to create new APIs and accept wallet/signer as prop, simplifying things

[ShookLyngs]

The first feature was shipped in #77, but the rest of them are not in the plan due to plan changes.