Git Flow: terraform-provider-coder: create new `coder_git_repo` data source

[johnstcn]

Create a new coder_git_repo Coder data source to support the following use-cases:

1. Pre-defined Git repo with hard-coded branch:

data "coder_git_repo" "repo" {
  provider = "my-github"
  url = "git@github.com:coder/coder.git"
}

module "git-clone" {
  url = data.coder_git_repo.repo.url
  branch_name = "main"
  git_providers = {
    "git@github.com": "my-github"
  }
}

2. Pre-defined Git repo with configurable branch:

data "coder_git_repo" "repo" {
  provider = "my-github"
  url = "git@github.com:coder/coder.git"
}

data "coder_parameter" "git_branch" {
  type = "string"
  default = data.coder_git_repo.repo.default_branch
}

module "git-clone" {
  url = data.coder_git_repo.repo.url
  branch_name = data.coder_parameter.git_branch.value
  git_providers = {
    "git@github.com": "my-github"
  }
}

2. Configurable Git repo with configurable branch:

data "coder_parameter" "git_repo" {
  type = "string"
  default = "git@github.com:coder/coder.git"
}

data "coder_git_repo" "repo" {
  provider = "my-github"
  url = data.coder_parameter.git_repo.value
}

data "coder_parameter" "git_branch" {
  type = "string"
  default = data.coder_git_repo.repo.default_branch
}

module "git-clone" {
  url = data.coder_git_repo.repo.url
  branch_name = data.coder_parameter.git_branch.value
  git_providers = {
    "git@github.com": "my-github"
  }
}

Implementation Notes:

• coderd will be responsible for querying the remote repository and setting the required data in the data source based on user inputs. We need to be using the existing user's oauth2 tokens to authenticate with the Git provider. Doing this in the provider means that typographical errors or issues cloning the repo will only be discovered at plan/apply time, and not while the user is still filling out the form.
• If possible, we should only allow one coder_git_repo data source per template.
• If possible, we should ensure that all coder_parameter references to this data source are immutable. Rationale: if a user later changes the git repo linked to their workspace, the expected behaviour is not clear.

[johnstcn]

After discussing with @Emyrk, we should hold off on adding this feature until the work on dynamic parameters reaches a more mature stage. Originally the hope was for this to be a new parameter type, but for various reasons this isn't straightforward in a non-hacky way.