Exempt issue/pr stale for specific author names

[kzu]

This allows excluding specific authors' issues/PRs. Useful for those created by project owners, for example.

Fixes #933

Description: This change adds the user: IUser property from the octokit issue to the IIssue interface so it can be checked against the new action option exempt-authors. It was implemented following the style from exempt-issue-labels approach, and two tests were added to cover both exempted and non-exempted author scenarios.

Readme docs already updated, so not sure if there are further docs changes to make.

Related issue: #933

Check list:

• [x] Mark if documentation changes are required.
• [x] Mark if tests were added or updated to cover the changes.