Extract MQTT client_id from client certificate and propagate to authnz backends

[MarcialRosales]

Proposed Changes

Address issue https://github.com/rabbitmq/rabbitmq-server/issues/9323

This plugin is accompanied by a docs PR: https://github.com/rabbitmq/rabbitmq-website/pull/1976

Types of Changes

What types of changes does your code introduce to this project? Put an x in the boxes that apply

• [ ] Bug fix (non-breaking change which fixes issue #NNNN)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• [ ] Documentation improvements (corrections, new content, etc)
• [ ] Cosmetic change (whitespace, formatting, etc)
• [ ] Build system and/or CI