Add DicussionCommentEvent Webhook

[apurwaj2]

This commit has changes to enable support to receive DiscussionComment Webhook Event, to fix google#2497 Changes include:

• Addition of event type for this type of event DiscussionCommentEvent
• Changes for identification and mapping of DiscussionCommentEvent
• Addition / Modification of relevant tests
• Addition of files created by go generate

[gmlewis]

Please run your files through "gofmt" as described in CONTRIBUTING.md.

[codecov]

Codecov Report

Merging #2678 (23a1718) into master (099fae4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2678   +/-   ##
=======================================
  Coverage   98.05%   98.05%           
=======================================
  Files         131      131           
  Lines       11442    11444    +2     
=======================================
+ Hits        11219    11221    +2     
  Misses        152      152           
  Partials       71       71           

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)

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[gmlewis]

Thank you, @apurwaj2 !
LGTM.

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

Note that I'm OOO until around March 6, so I may not be able to merge this before then if we don't get an approval shortly.

[apurwaj2]

Thank you, @apurwaj2 ! LGTM.

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

Note that I'm OOO until around March 6, so I may not be able to merge this before then if we don't get an approval shortly.

Sure. Thanks, for approving.

[gmlewis]

Thank you, @valbeat !
Merging.