Add missing fields to emoji and milestone event types

Summary

GitLab's webhook payloads for emoji and milestone events include undocumented fields for various awardable types. This MR enhances the event types to support these fields.

Changes

EmojiEvent - Source: lib/gitlab/data_builder/emoji.rb

• Add MergeRequest field
• Add ProjectSnippet field
• Add Commit field for notes on commits
• Change Project type from EmojiEventProject to EventProject

MilestoneWebhookEvent - Source: lib/gitlab/data_builder/milestone.rb

• Add Group field
• Change Project type from MilestoneEventProject to *EventProject

Shared types - Same hook_attrs methods reused across multiple webhooks:

Type	Definition	GitLab Usage (2 per type)	client-go Usage
EventProject	app/models/project.rb:2416	emoji.rb:36, milestone.rb:24	EmojiEvent.Project, MilestoneWebhookEvent.Project
EventGroup	app/models/group.rb:1247	milestone.rb:25, resource_access_token_payload.rb:13	MilestoneWebhookEvent.Group, GroupResourceAccessTokenEvent.Group
EventMergeRequest	app/models/merge_request.rb:939	emoji.rb:21, note.rb:52	EmojiEvent.MergeRequest
EventSnippet	app/models/snippet.rb:256	emoji.rb:21, note.rb:54	EmojiEvent.ProjectSnippet
EventCommit	app/models/commit.rb:282	emoji.rb:14-15, note.rb:81	EmojiEvent.Commit

API Breaking Change Considerations

- EmojiEvent.Project type changed from EmojiEventProject to EventProject - MilestoneWebhookEvent.Project type changed from MilestoneEventProject to *EventProject - GroupResourceAccessTokenEvent.Group type changed from GroupResourceAccessTokenEventGroup to EventGroup - EmojiEventProject struct removed - MilestoneEventProject struct removed - GroupResourceAccessTokenEventGroup struct removed

This MR contains no breaking changes. All existing struct types are preserved to maintain backward compatibility:

• EmojiEvent.Project remains EmojiEventProject
• MilestoneWebhookEvent.Project remains MilestoneEventProject (value type)
• GroupResourceAccessTokenEvent.Group remains GroupResourceAccessTokenEventGroup

Notes

According to GitLab's milestone.rb, the Project field is optional (uses milestone.project&.hook_attrs), as group milestones do not belong to a project. However, MilestoneWebhookEvent.Project remains a value type to maintain backward compatibility.