[alias-server] Use helmet to improve express security

Summary:
Per express docs security improvement recommendations, implement helmet in alias-server

Ref https://expressjs.com/en/advanced/best-practice-security.html
Ref https://www.npmjs.com/package/helmet

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14639