[Cashtab][Alias] Implement getAliasDetails

Summary:
T3216

Add the new getAliasDetails function in aliasUtils.js to wrap around the fetch call to alias-server's alias endpoint.

This will be used for retrieving alias info (associated address, txid, blockheight) and also to check whether the alias is registered or not.

Test Plan:
npm test
npm start and sanity check Cashtab doesn't blow up with alias flag remaining as false

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D14191