Add type and type tests to GraphQL resolvers

What does this MR do?

Adds declared type to GraphQL resolvers plus tests.

In order to correctly test field authorisations (in preparation for when we do use the graphql-ruby gem authorisation), we need to be able to test auth properly by building an arbitrary field from a resolver. That will come in a subsequent MR.

Resolvers should never be re-used so it's okay to declare a type specific to the usage of a single field on a single type.

Screenshots (strongly suggested)

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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