Add metrics dashboard feature CRUD permissions to projects policy

What does this MR do?

During discussion at #233561 (comment 393037545) lack of mange permissions for metrics dashboard was identified. This MR adds create update and delete permissions to metrics dashboard feature. Because right now changes to metrics dashboards are made by interacting with contents of .gitalb/dashboards directory within project repository, enabling newly added permissions is dependent on ability to push_code to repository.

This new permissions haven't been used anywhere yet, therefore I've not added changelod entry.

Does this MR meet the acceptance criteria?

Conformity

- [ ] Changelog entry

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides