Use Issues/Timelines API to retrieve all previously requested reviewers

[jamoor-moj]

The previous PR (#3) attempted to skip re-assigning aliases that had previously been assigned to the pull request.

Based on the github documentation, the previous API will only return current aliases that have been requested for review. This breaks two scenarios:

1. user aliases that have previously signed off will not be returned (and therefore re-added)
2. team aliases that have been signed off "on behalf of" a user alias will not be returned (they get removed entirely from the PR) (and therefore re-added).

This PR moves to using the timelineevents API which will give us every alias that has been requested regardless of their current status. Because the timeline can have a lot of events and the REST version doesn't allow us to filter, I opted to both swap to using GraphQL version and added pagination support..

The final change is to Unit Tests where we pull in rewire in order to mock private methods of our github.js file. Primarily due to issues with mocking a constructor now that we had to change the instantiation of octokit.

[Warwolt]

I have some minor thoughts but otherwise I think this looks good to me (but I'm also not a GHA hacker so, would do good for others to give reviews as well)