Add validation VS insertion mechanism

### Description
As per the changes in [this PR](https://github.com/dotnet/project-system/pull/8420), the validation PR creation mechanism is no longer present. This is not complicated to add since the `official.yml` pipeline already does insertions. However, we need to modify it to do the following:
- On PR merge, run the Insertion stage in a 'validation mode'
- Allow the 'validation insertion' to be triggered via pipeline parameter
- Modify the Insertion stage to change the `InsertionPayloadName` to contain `[VALIDATION]` while in validation mode
- ~*[Most of the work]*~ Create a step that runs after the insertion is created that marks the insertion PR as 'Draft'
- ~Likely involves an AzDO API call~
- The MicroBuild insertion task now supports a flag to mark the insertion as Draft

With this changes this allows:
- Manual queuing of validation insertions
- Automatic queuing of validation insertions on PR merge

### Stretch Goal
- Mechanism to clean up validation insertions automatically
- Could run as part of nightly build as a separate stage