Adds mTLS support for GitRepositories

[benke33]

Hi,

Here's an updated version of PR #1302, without the proxy parts.

This PR adds mTLS support for GitRepositories, which is required if you have your git infrastructure
behind a firewall.
With this feature you don't need any additional setup like nginx reverse proxy or mtls-transmitter,
to access git repos behind a firewall requiring mTLS.

Certificates are added to the same secret used for the credentials in spec.secretRef, CA is optional:

apiversion: v1
kind: Secret
metadata:
  name: my-mtls-secret
stringData:
  certFile: < certificate data >
  keyFile: < private key data > 
  caFile: < CA data >
  username: < git username >
  password: < git token >
  or
  bearerToken: < git token >

Credit to @VasylR for the PR #1302

This PR closes #1761