Model Experiments: Add Experiments to GraphQL

What does this MR do and why?

This merge request introduces GraphQL support for machine learning experiments in GitLab. It adds new types, resolvers, and fields to handle ML experiments, including the ability to query individual experiments and lists of experiments. The changes include new GraphQL types for ML experiments, resolvers to fetch experiment details and lists, and updates to existing types to incorporate the new ML experiment fields.

References

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Before	After

How to set up and validate locally

1. In Rails console create a model with experiment and candidates:

p = Project.find_by_id(7)
m = Ml::CreateModelService.new(p, "model_1").execute
20.times { |i| Ml::CreateCandidateService.new(m.default_experiment, {}).execute }

1. Open GraphQL explorer: http://127.0.0.1:3000/-/graphql-explorer
2. Fetch all experiments for a project

query getExperiments($fullPath: ID! = "flightjs/Flight") {
  project(fullPath: $fullPath) {
    id
    mlExperiments {
      nodes {
        id
        name
        createdAt
        path
        candidateCount
        creator {
          id
          name
          webUrl
        }
        modelId
        candidates {
          nodes {
            id
            name
            createdAt
          }
        }
      }
    }
  }
}

1. Fetch a specific experiment

query getExperiment {
  mlExperiment(id: "gid://gitlab/Ml::Experiment/3") {
    id
    name
    createdAt
    path
    modelId
    candidateCount
    creator {
      id
      name
      webUrl
    }
    modelId
  }
}

Numbered steps to set up and validate the change are strongly suggested.

Related to #509320 (closed)