Add payload for discussion_comment event

[gsmet]

Description

Long time no see :).

Add payload for discussion_comment event.
I already added discussion a while ago and someone in our team had a use case for discussion_comment.

Before submitting a PR:

• Changes must not break binary backwards compatibility. If you are unclear on how to make the change you think is needed while maintaining backward compatibility, CONTRIBUTING.md for details.
• Add JavaDocs and other comments explaining the behavior.
• When adding or updating methods that fetch entities, add @link JavaDoc entries to the relevant documentation on https://docs.github.com/en/rest .
• Add tests that cover any added or changed code. This generally requires capturing snapshot test data. See CONTRIBUTING.md for details.
• Run mvn -D enable-ci clean install site locally. If this command doesn't succeed, your change will not pass CI.
• Push your changes to a branch other than main. You will create your PR from that branch.

When creating a PR:

• Fill in the "Description" above with clear summary of the changes. This includes:
  • If this PR fixes one or more issues, include "Fixes #" lines for each issue.
  • Provide links to relevant documentation on https://docs.github.com/en/rest where possible. If not including links, explain why not.
• All lines of new code should be covered by tests as reported by code coverage. Any lines that are not covered must have PR comments explaining why they cannot be covered. For example, "Reaching this particular exception is hard and is not a particular common scenario."
• Enable "Allow edits from maintainers".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fa675f3) 80.41% compared to head (4932a7f) 80.44%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1781      +/-   ##
============================================
+ Coverage     80.41%   80.44%   +0.02%     
- Complexity     2309     2316       +7     
============================================
  Files           218      219       +1     
  Lines          7016     7026      +10     
  Branches        371      371              
============================================
+ Hits           5642     5652      +10     
  Misses         1141     1141              
  Partials        233      233              

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[gsmet]

Thanks @bitwiseman !