Feature request: Support for external authentication mechanism about amqprs

[gftea]

The tls example use self signed Root certificate. Have you read through https://www.rabbitmq.com/ssl.html#peer-verification?
Meanwhile check the guidelines of using openssl for troubleshooting

[tetofonta]

Yes i can get tls working and client certificate verification is ok, but the library still tries to authenticate the user with the username and password instead of specifiying EXTERNAL authentication and use rabbitmq_auth_mechanism_ssl.

Looking at the source code, External auth method is commented in the Authentication method struct at https://github.com/gftea/amqprs/blob/92da1bba56b9177876a6d20ecd8d74cbaa2cc9ad/amqprs/src/api/security.rs#L27C12-L27C12

[gftea]

@tetofonta , I think it should be trival to change, add EXTERNAL variant, and have an api in SecurityCredentials to support this new auth mechanism
in get_response, just a empty string might work.
you like to create a PR to try out?