[lbussell]

Fixes https://github.com/dotnet/dotnet-docker-internal/issues/6741.

Summary of changes:

• Automatically trigger the update-dependencies-internal-official pipeline every time there is a new staged .NET version.
• Only use service connections/managed identity for submitting PRs to Azure DevOps.
• Both official and unofficial pipelines use pipeline resources to reference the staging pipeline, making them more similar.
• Use the Darc backend to submit pull requests in the from-staging-pipeline command.
• from-staging-pipeline command now has two explicit "modes" - Local and Remote.
  • Local makes changes in your working repo. This is for the local dev workflow.
  • Remote can target any branch and does not touch your local git repo. It works in a temporary git repo. It always submits a pull request. This is the same way the sync pipeline works. Example scenarios where this is useful:
    • From your local machine, run one command to submit an update PR to any internal branch without touching your local repo.
    • Use the latest code from the nightly branch to update dependencies in any other internal branch.
  • All the git functionality is encapsulated in the private GitRepoContext class so that it required minimal changes to the logic in FromStagingPipelineCommand.Execute.
• Share some code between FromStagingPipelineCommand and SyncInternalReleaseCommand

Example pull request generated by these changes: https://dev.azure.com/dnceng/internal/_git/dotnet-dotnet-docker/pullrequest/54519