Extend az repos pr reviewers to differentiate between optional and required reviewers

**Related command**
```
az repos pr reviewer
az repos pr create --reviewers
```

**Is your feature request related to a problem? Please describe.**
In my organization, we use pipelines to create a specific branch in a repository. I'd like to add the executing person to the newly created branch as a required reviewer, so that he is aware of his current required PRs.

**Describe the solution you'd like**
I would like to have the option whether I want to add reviewers as required or optional, maybe using a different parameter like:
`az repos pr create --required-reviewers`
`az repos pr reviewer --required-reviewers`
so that the current functionality is not broken.

**Describe alternatives you've considered**
Currently the only workaround I consider is capturing the REST request and executing it myself.

**Additional context**