Add category resolver

What does this MR do and why?

Adds category type to group type, together with a category resolver. The resolver returns the existing categories, with fallback to the default values. In case the default values are used, it fills the namespace_id of the categories and attributes with the related group_id.

Changelog: added
EE: true

How to set up and validate locally

1. Enable security_categories feature flag:
   Feature.enable(: security_categories_and_attributes)
2. Use this GraphQL query to get the default values:

    query categories($fullPath: ID!) {
      group(fullPath: $fullPath) {
        id,
        securityCategories{
          name
          description
          editableState
          id
          multipleSelection
          templateType
          securityAttributes{
            name
            color
            description
            editableState
            id
          }
        }
      }
    }

3. Use this mutation to create a category:

     mutation verify {
       securityCategoryCreate(input: {
         namespaceId: "gid://gitlab/Group/<GROUP_ID>",
         name:"This is a test",
         description:"This is a desc",
         multipleSelection: true,
       }){
         securityCategory {
           id
           name
           description
         }
       }
     }

4. Repeat step 2 to see the new category, together with the default values.

MR acceptance checklist

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Related to Add security category type to group (#562743 - closed) • Gal Katz • 18.4