Calculate the advanced and direct update URLs for the key discovery
portion[1] of the OpenPGP Web Key Directory specification, and include
the URLs in the key listing in ostree_repo_remote_get_gpg_keys(). These
URLs can be used to locate updated GPG keys for the remote.
1. https://datatracker.ietf.org/doc/html/draft-koch-openpgp-webkey-service#section-3.1