Implement TOFU
Since Gemini servers often have self-signed (or even expired) certificates, and since the specification suggests to use TOFU, it would be a nice improvment.
This does not fit well the Nagios framework, where requests are idempotent. Suggestion:
- define a location on the disk to put the fingerprint of the cert
- if tofu=True, store and check the fingerprint
- if it changes, exit CRITICAL with a message explaining how to reset the fingerprint.