To allow Dapps users to sign a transaction in the qubic wallet through wallet connect

[sallymoc]

Method name
qubic_signTransaction

Purpose
Allows dApps to request a transaction to be signed without sending it.

Usage
Input should be the same to the used for "qubic_sendTransaction".
The wallet will return a signed transaction object, allowing the dApp to handle broadcasting it to the network later (if necessary).