[alias-server] Add API endpoint to get pending aliases by address
Summary: Expose /pending/<address> endpoint for use in Cashtab to prevent accidental double registration of aliases and track user's pending aliases to success / failure
Test Plan: npm test
Reviewers: #bitcoin_abc, emack, Fabien
Reviewed By: #bitcoin_abc, emack, Fabien
Subscribers: Fabien, emack
Differential Revision: https://reviews.bitcoinabc.org/D14488