Allow filtering work items by Key Result type

Related to #385084 (closed)

What does this MR do and why?

Adds the ability to filter work items by type KEY_RESULT using GraphQL.

How to set up and validate locally

1. Enable feature flag in rails console

   Feature.enable(:okrs_mvc)

2. Visit http://gdk.test:3000/-/graphql-explorer and create work items with different types:

create work item mutations

mutation createKeyResult {
  workItemCreate(
    input: {title: "Key Result 1", projectPath: "gitlab-org/gitlab-test", workItemTypeId: "gid://gitlab/WorkItems::Type/2417"}
  ) {
    workItem {
      title
    }
    errors
  }
}

mutation createObjective {
  workItemCreate(
    input: {title: "Objective 1", projectPath: "gitlab-org/gitlab-test", workItemTypeId: "gid://gitlab/WorkItems::Type/2416"}
  ) {
    workItem {
      title
    }
    errors
  }
}

mutation createIssue {
  workItemCreate(
    input: {title: "Issue 1", projectPath: "gitlab-org/gitlab-test", workItemTypeId: "gid://gitlab/WorkItems::Type/1"}
  ) {
    workItem {
      title
    }
    errors
  }
}

3. Query the project's work items using the types filter:

query keyResults {
  project(fullPath: "gitlab-org/gitlab-test") {
    workItems(types: KEY_RESULT) {
      edges {
        node {
          title
        }
      }
    }
  }
}

4. The response should only contain key results

{"data": {"project": {"workItems": {"edges": [{"node": {"title": "Key Result 1"}}]}}}}

MR acceptance checklist

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