feat: add mtls support resolves #37

[rtoma]

Description of your changes

My stab at adding mTLS support, re https://github.com/crossplane-contrib/provider-kafka/issues/37

Tested with AWS MSK, using this ProviderConfig credential Secret:

{
  "brokers": [
    "b-1.redacted.kafka.eu-central-1.amazonaws.com:9094",
    "b-2.redacted.kafka.eu-central-1.amazonaws.com:9094",
    "b-3.redacted.kafka.eu-central-1.amazonaws.com:9094"
  ],
  "tls": {
    "clientCertificateSecretRef": {
      "name": "my-client-cert",
      "namespace": "some-ns"
    }
  }
}

Where clientCertificateSecretRef references a TLS Secret by name and namespace. The Secret keys used match the TLS definition, as used by cert-manager.

I have:

• [X] Read and followed Crossplane's contribution process.
• [X] Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

• Test on AWS MSK with AWS Private CA linked
• Client certificate requested via cert-manager, signed by AWS Private CA using https://github.com/cert-manager/aws-privateca-issuer
• ProviderConfig credential Secret as described above with ref to the client cert Secret.