Add support for mTLS to GitHub App transport

[abhijith-darshan]

notification-controller currently generates a TLS config for the underlying HTTP transport if Provider spec contains a certSecretRef.

Related to the following issues, notification-controller will also use the TLS config for mTLS GitHub App scenarios as well -

• git/github: Add support for mTLS to GitHub App transport pkg#999

• Add support for mTLS to GitHub App transport source-controller#1860

• Add support for mTLS to GitHub App transport image-automation-controller#947

• .spec.certSecretRef is the default choice for extracting the ca.crt.

• If the ca.crt is present in both .spec.certSecretRef and .spec.secretRef then .spec.certSecretRef takes the higher precedence.

• The ca.crt in .spec.secretRef is only consider if the provider is of one of the git types and certSecretRef is not specified in the spec.

example:

apiVersion: notification.toolkit.fluxcd.io/v1beta3
kind: Provider
....
spec:
  address: https://my.company.corp/org/repo
  certSecretRef:
    name: some-ca-secret # TLS config with custom ca is extracted from this secret
  secretRef:
    name: github-app-ca-secret # the ca.crt if present in this secret reference will be ignored
  type: github

TODOS:

• ca.crt for TLS config should be generated from .spec.certSecretRef and if not specified then it should attempt to generate TLS config from .spec.secretRef (only if the provider is one of the git types)
• Add relevant test cases
• Update the docs regarding the ca.crt preference

[matheuscscp]

First pass 👌

[matheuscscp]

This is looking good, thanks! Please add docs and amend the commit message to match the PR title 🙏

[matheuscscp]

In the docs we must also mention that if a client cert is present, it will be used (just like we mentioned in the GitRepo docs)

[matheuscscp]

LGTM! 🚀