Support querying files changed by a PR.

**Is your feature request related to a problem? Please describe.**
I use 'az repos pr' to get pull request stats, but see no way to list out the files changed (additions/deletions).

**Describe the solution you'd like**
If this feature is already supported, please provide a link to the documentation. Otherwise, implement a similar functionality to the GitHub CLI command `gh pr view --json changedFiles,files`.