[alias-server] Return price and price expiration blockheight from /alias endpoint for unregistered aliases

Summary: This approach simplifies implementing registration in wallets. Wallets already need to make an API call to see if an alias is available or not. If it is not available, return the price (and the next block, and the blockheight this price is good thru).

Test Plan: npm test

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: Fabien, emack

Differential Revision: https://reviews.bitcoinabc.org/D14268