split sign method for TonBaseStaker

[mkaczanowski]

TL;DR

The sign method does a few other things (like wallet creation etc) prior calling the signer. If SDK user wants to skip the signer, he/she would have to replicate some of the logic which ain't great.

This PR splits the sign method into:

1. prepareSigningData - prepare the final transaction
2. prepareSignerTx - turns signed TxCell into a SignedTx structure

Example without signer abstraction:

const staker = TonPoolStaker(...)
const externalCustomSigner = signer({ ... })
const { signerAddress, signerPublicKey } = signer({...})

const signingData = await this.prepareSigningData({
  signerAddress,
  signerPublicKey,
  tx: await staker.buildStakeTx(...)
})

// sign transaction via custom signer or call some method that returns signed cell
const signedTxCell = await externalCustomSigner.sign(signingData.txCell)

const signedTx = await this.prepareSignedTx({
  signerAddress,
  signedTxCell,
  stateInit: signingData.txArgs.stateInit
})

staker.broadcast({ signedTx })

Test Plan

https://testnet.tonviewer.com/transaction/58a1db257f9309c587063f9439cbfef8738b6b52fcc2dafa2592e220d7f30996

[welldan97]

Looking great, we support multistep signing now