Add AI catalog item consumer create mutation

What does this MR do and why?

We need a way to enable Ai catalog flows in groups and projects.

This MR introduces the service and mutation to support that.

References

Screenshots or screen recordings

How to set up and validate locally

1. Enable global_ai_catalog feature flag
2. Create a flow:

    mutation {
      aiCatalogFlowCreate(
        input: {
          projectId: "gid://gitlab/Project/1000000"
          name: "Test"
          description: "Test"
          public: true
        }
      ) {
        item {
          id
        }
        errors
      }
    }

3. Create a consumer:

    mutation {
      aiCatalogItemConsumerCreate(
        input: {
          itemId: "gid://gitlab/Ai::Catalog::Item/45"
        	target: {
            projectId: "gid://gitlab/Project/1000000"
          }
        }
      ) {
        errors
      }
    }

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Related to #554268 (closed)