[Cashtab][Alias] Update alias-server wrappers to return JSON

Summary: As per feedback from D14212, the wrapper for the alias-server endpoints should be returning the JSON responses directly rather than leaving it to frontend components to parse.

Test Plan: npm test

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14219