Check that we conform to the OpenADR 3.1 TLS guidance

[bjorn3]

• Only allow tls 1.2+ (I think already implicitly done due to rustls usage, which only supports tls 1.2+).
• Must not use unencrypted http.
• Have a configuration option to allow self-signed certificates (optional. if we don't have such an option, self-signed certificates should be denied, even if the spec does not require this)