[alias-server] [p17] Add API endpoint, MVP of app

Summary:
T2922

Depends on D13162

Add an API endpoint that delivers a JSON of valid alias-address pairs

Test Plan:
node index.js
Navigate to localhost:5000/aliases in browser
Should see aliases

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D13165