[alias-server] Add function to query database by alias

Summary:
T3060

Add new database function to lookup a valid alias registration by address. This function will be implemented in a later diff to create an API endpoint used to check if a given alias is available to be registered.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14097