Add webhook processor for projects_v2_item and more payload schema

[ithielnor]

Resolves #657
Resolves #659

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Before the change?

• There is no method on WebhookEventProcessor to override to handle projects_v2_item events.
• The ProjectsV2ItemEvent.Changes does not contain the field name or other new fields: https://github.blog/changelog/2024-06-27-github-issues-projects-graphql-and-webhook-support-for-project-status-updates-and-more/#using-webhooks-for-project-custom-field-changes

After the change?

• There is a new ProcessProjectsV2ItemWebhookAsync method on WebhookeventProcessor which can be overridden to handle projects_v2_item events.
• The ProjectsV2ItemEvent.Changes now contains FieldName and other new schema

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

• Yes
• No

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[github-actions]

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