Add private extension repository support

Adding support to use a private extension repository, to use it we just need to add the following environment variable at the organization level.

PRIVATE_EXTENSION_VCS_ID_AUTH

The above will require the vcs id to use when authentication to the private extension repository

For example when using the helm chart we can set the private extension repository like the following:

executor:
  properties:
    toolsRepository: "https://myprivate.com/secret/my-private-repository.git"
    toolsBranch: "main"

Fix #928

AzBuilder / terrakube