Add a cop to check for GraphQL field descriptions

What does this MR do?

Adds a Rubocop custom cop to raise an error if a modified GraphQL field is missing a description. Since descriptions are a part of the generated GraphQL docs, having the descriptions filled in makes it more useful.

This cop ignores existing failures, but #67087 addresses that.

EE: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/16438

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Performance and Testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/64322