Create pipeline template that blocks merging to main for private repos

For our private repos we create a one-way sync between public main to private main and as such we should not allow any merging of PRs into the private main. We should create a simple template that we can create a pipeline for in the private repos to enforce this. Something similar to https://github.com/Azure/azure-rest-api-specs/blob/main/azure-pipelines.yml (https://github.com/Azure/azure-rest-api-specs/blob/main/.azure-pipelines/BranchProtectionForPrivateRepo.yml) except we should refactor that to make it more generically work for all our private repos and also update the pipeline in the specs repo to use that same template.

FYI @raych1 eventually we will replace the one in the specs repo with a more generic version and share that version across all our private repos.