Add support for Parity Signer

Parity signer is a mobile app that lets you sign/submit QR-encoded transactions. Supporting it means supporting basically any QR-based signer, which is a nice, cheap key isolation strategy.

You can read about MyCrypto's integration here:
https://paritytech.io/send-transactions-with-mycrypto-beta-and-parity-signer/

Start by reading the linked MyCrypto integration article and the Parity Signer description to understand the QR-encoded transaction flow. Then locate the extension’s transaction-signing entry point and define the supported QR exchange and completion behavior; the issue names no files, tests, or acceptance criteria.

Written by the indexing model from the issue text.