chore(ci): add workflow to validate gpg/ folder content

[aduh95]

• add generate-gpg-dir.sh script – mostly useful for CI
• add workflow that fails if the commited keyrings are not in sync with the README.
• a check that validates all releases signature can be verified with the pubring

What's missing:

• add gpg-only-active-keys folder, for folks who only want to check "future" releases rather than past ones. moving this to feat: add gpg-only-active-keys/ keyring #38
• a check to ensure README is in sync with nodejs/node READMEchore(ci): validate README lists of keys #39
• a check to ensure latest versions of all non-EOL release lines can be verified with the "only-active-keys" keyringchore(ci): validate non-EOL releases are signed with an "active" key #43
• a check to ensure we can download "active" keys from well-known public key servers (keys.openpgp.org, keyserver.ubuntu.com, pgp.mit.edu) – not possible atm given my former public key is no longer retrievable from openpgp.org

[aduh95]

@nodejs/releasers can I get some reviews please?