Add support for the pull request review thread event

[taisph]

Fixes#2324

[codecov]

Codecov Report

Merging #2326 (0f34008) into master (c28ccfd) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@ Coverage Diff @@## master #2326 +/- ##
=======================================
Coverage 98.04% 98.04% =======================================
Files 117 118 +1 Lines 10454 10458 +4 =======================================
+ Hits 10250 10254 +4 
Misses 140 140 Partials 64 64 

Impacted Files	Coverage Δ
github/event_types.go	100.00% <ø> (ø)
github/messages.go	100.00% <ø> (ø)
github/event.go	100.00% <100.00%> (ø)
github/pulls_threads.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c28ccfd...0f34008. Read the comment docs.

[taisph]

I added the link https://docs.github.com/en/developers/webhooks-and-events/webhook-events-and-payloads#pull_request_review_thread which presumably will be the correct link once they update the docs by looking at the existing structure. I don't know what to expect with regards to when they'll update it as I managed to find a misnamed issue about the missing documentation that was created back in November 2021.

Should this PR be kept back until the docs are updated?

[gmlewis]

Should this PR be kept back until the docs are updated?

I don't think that's necessary. Many endpoints have been added in the past just based on observing the response received, so I think this is fine. It can always be updated once the official docs are released. Thank you, @taisph !

[gmlewis]

Thank you, @taisph !
LGTM.

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

[raynigon]

LGTM

[gmlewis]

Thank you, @raynigon !
Merging.