Allow tags for blueprint VCS source

Right now only branches are supported:

vcs: branch: branch-name namespace: namespace-name repository: repo-name provider: AZURE_DEVOPS

We are planning to use blueprints to allow developers to deploy infrastructure based on our reusable Terraform modules. Our reusable modules use tagging for versioning, and so we would like our blueprints to target a specific tag so that when we merge breaking module changes to the main branch, we do not have to update our blueprints immediately.