Add support for "performed_via_github_app" and "requested_team" in IssueEvent

[Shyclyde]

Issue Source

#2945 mentions the reasoning for this PR.

Why

"performed_via_github_app" and "requested_team" are valuable results in the issue events response (when available) to determine more about the event. It also keeps the struct up to date with the API response.

What

• Adds these values to the IssueEvent struct.
• Adds test cases for added values.
• Restructures relevant test cases slightly to match the order of the given/want for easier reference.

Other notes

Ran all available test scripts, updated accordingly. Tests passing.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gmlewis]

Thank you, @Shyclyde !
LGTM.

Awaiting second LGTM+Approval from any other contributor to this repo before merging.

[codecov]

Codecov Report

Merging #2946 (954e7ed) into master (8cd452b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2946   +/-   ##
=======================================
  Coverage   98.19%   98.19%           
=======================================
  Files         145      145           
  Lines       12720    12720           
=======================================
  Hits        12490    12490           
  Misses        156      156           
  Partials       74       74           

Files	Coverage Δ
github/event_types.go	100.00% <ø> (ø)
github/issues_events.go	100.00% <ø> (ø)
github/issues_timeline.go	100.00% <ø> (ø)

[gmlewis]

Thank you, @WillAbides !
Merging.