Separate out into nested modules for git provider specific support

[Dev25]

For better git provider support where we want to automatically create SSH keys, break down this module into read only public repos (root module) + provider specific nested modules.

/  # Read only flux deployments using public repos
/modules/github/
/modules/gitlab/ 

As long as git providers have a terraform provider and they support creating SSH/deploy keys to grant access, it can then be a nested module option.