[alias-server] Helper function to get array of strings with just alias names

Summary:
T3060

Depends on D13720

Helper function to prepare param necessary for getValidAliasRegistrations to work with partial tx history

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13721