[alias-server] [p8] Complete getAliases function

Summary:
T2922

Depends on D13080

Complete getAliases function, with unit test

Test Plan:
node index.js and confirm output matches latest
npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13084