feat(rgw/kafka): add mTLS client certificate authentication for Kafka notifications

feat(rgw/kafka): add mTLS client certificate authentication for Kafka notifications
Add support for mutual TLS (mTLS) client certificate authentication
when publishing bucket notifications to Kafka brokers. RGW can now
present a client certificate and private key to authenticate with
brokers that require ssl.client.auth=required.
Changes:
- Add ssl-certificate-location, ssl-key-location, and ssl-key-password
  topic attributes for configuring client certificates
- Validate that ssl_certificate and ssl_key are provided together
- Include ssl_key_password in connection identity (hash/equality)
- Add kafka-security.sh script for generating broker and client TLS certs
- Add mTLS test (test_notification_kafka_security_ssl_mtls) using
  use_mtls=True flag on the existing SSL security path
- Update RGW notifications documentation with mTLS parameters

Fixes: http://tracker.ceph.com/issues/67427
Signed-off-by: Jan Radon <jan.fabian.radon@sap.com>