Git Flow: coder/modules: create unified `git-repo` module

Builds upon https://github.com/coder/internal/issues/407 https://github.com/coder/internal/issues/406 https://github.com/coder/internal/issues/405 https://github.com/coder/internal/issues/404

Create a new unified `git-flow` module that comprises the functionality of both the existing `git-config` and `git_clone` modules:

```
data "coder_external_auth" "github" {
id = "github"
}

module "git-repo" {
# Base directory to clone the repository
dir = "$HOME"
# Set GitHub as authentication method (Optional, required for private repos)
auth = data.coder_external_auth.github.id
# URL for a repository. When the url is set, the user is not prompted to fill in a repo
url = "git@github.com:coder/coder.git"
# Branch, tag or commit hash. When ref is set, the autocomplete will not allow the user to pick a ref
ref = "main"
```