Adds grapqhl mappings for Ml::Candidate

What does this MR do and why?

Adds grapqhl mappings for Ml::Candidate

First mappings for Candidate type, to be used in the model registry.

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:model_registry)

2. Create a model and a bunch of candidates

   p = Project.find_by_id(1)
   m = Ml::FindOrCreateModelService.new(p, "model_1").execute
   20.times { |i| Ml::CreateCandidateService.new(m.default_experiment, {}).execute }

3. Open /-/graphql_explorer, the following query should work (replace MODEL_ID):

    {
      mlModel(id: "gid://gitlab/Ml::Model/MODEL_ID") {
        id
        candidates(first: 10) {
          count
          nodes {
            id
            name
            createdAt
            _links {
              showPath
            }
          }
          pageInfo {
            hasNextPage
            hasPreviousPage
            endCursor
            startCursor
          }
        }
      }
    }

Database

Ml::Candidate.without_model_version.take(10)

SELECT
    "ml_candidates".*
FROM
    "ml_candidates"
WHERE
    "ml_candidates"."model_version_id" IS NULL

Seq Scan on ml_candidates  (cost=0.00..5.62 rows=155 width=139) (actual time=0.015..0.084 rows=155 loops=1)
  Filter: (model_version_id IS NULL)
  Rows Removed by Filter: 7
Planning Time: 1.944 ms
Execution Time: 0.358 ms

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.