GraphQL: Add BoardType and resolver

What does this MR do?

This is the initial MR for adding a query for project/group boards. It adds the basic type and resolver. This was pulled out of !22497 (merged) in order to make each piece smaller.

I've only added the specs for the resolver. Specs for the type will be added in !22497 (merged) (as I was not able to test the actual type without adding it to the Project or Group schema).

Follow on MR is !22497 (merged)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability 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