authenticate using pubkey before making requests

There should be some kind of authentication, otherwise the endpoint would be prone to spam requests once made live on `api.getalby.com`

Start by locating the Go HTTP endpoint implementation and reviewing how requests are currently handled; the issue names no file, test, or entry point. Clarify the public-key authentication scheme and its acceptance criteria, with completion defined as authenticated requests being accepted and unauthenticated or abusive requests being prevented.

Written by the indexing model from the issue text.