Add no-op arguments to item consumer mutations

What does this MR do and why?

This is just so the FE can get a head start on implementing the FE. The BE implementations will follow.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

Confirm that the requests still work with and without the new arguments without causing any issues:

mutation {
  aiCatalogItemConsumerUpdate(
    input: {id: "gid://gitlab/Ai::Catalog::ItemConsumer/19", pinnedVersionPrefix: "1.0.0", serviceAccount: "gid://gitlab/User/1000000", triggerTypes: []}
  ) {
    errors
    itemConsumer {
      pinnedVersionPrefix
    }
  }
}

mutation {
  aiCatalogItemConsumerCreate(
    input: {itemId: "gid://gitlab/Ai::Catalog::Item/76", target: {groupId: "gid://gitlab/Group/1000000"}, createServiceAccount: true, serviceAccountName: "Test", parentItemConsumerId: "gid://gitlab/Ai::Catalog::ItemConsumer/10", triggerTypes: []}
  ) {
    itemConsumer {
      id
    }
    errors
  }
}

And try again without the createServiceAccount, serviceAccountName, parentItemConsumerId serviceAccount and triggerTypes arguments, to ensure it hasn't broken anything.

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.