Create a way for the advanced transaction builder to communicate with WalletConnect for signing

[rkalis]

No description provided.

[mr-zwets]

this is what the signTransaction interface of BCH WalletConnect looks like

signTransaction: (
  options: {
    transaction: string | TransactionBCH,
    sourceOutputs: (Input | Output | ContractInfo)[],
    broadcast?: boolean,
    userPrompt?: string}
) 
=> Promise<{ signedTransaction: string, signedTransactionHash: string} | undefined>;

( see https://github.com/mainnet-pat/wc2-bch-bcr )

So the main way the transactionbuilder/cashscript could help is by constructing the sourceOutputs object in the required format