pgjwt should support asymmetric/RSA private key signing

[raphet]

See https://security.stackexchange.com/a/234018

Having multiple services accessing a public key for signage verification - with postgres as authority server is a safer situation than distributing the secret HMAC key on every services.

Therefore I am suggesting to include into the pgjwt module a way to support creating JWT using a private RSA key within postgres. That way, login/authentication via postgrest would be possible and third party services would only need to know the public key to verify any jwt token's origin/legitimacy.

[michelp]

Unfortunately pgcrypto does not support public key signing.