Add subcommand for `az repos pr` to list/add/delete tags

**Related command**
`az repos pr`

**Is your feature request related to a problem? Please describe.**
I want to use the az cli in my CI/CD pipeline to read tags, but I have to use the REST api instead. It would be great if I could read the tags directly

**Describe the solution you'd like**
`az repos pr tags --id 2582 --detect true`, where `command` is list, add, and delete, and (for add and delete) takes an additional parameter which is the string of the tag to manipulate.

**Describe alternatives you've considered**
I can currently list tags from the REST api, but that is cumbersome and not ideal when I can do other operations directly in the cli.

**Additional context**
none