Support `discussion_id` field in commit and snippet comment webhook

vntwrequested to merge
vntw/client-go:add-note-wh-fields into main

This adds support for the discussion_id field in both commit and snippet webhook events, MR and issue comment events already support it.

The updated note fixtures are done according to https://gitlab.com/gitlab-org/gitlab/-/blob/aedc40e2dacda79d7bdcbe2efe48606544f5a5f0/lib/gitlab/hook_data/note_builder.rb and real responses from Gitlab.com webhooks to be consistent.

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