batches: create mutation for upserting batch changes

Created by: BolajiOlajide

Part 1 #780

Test plan

• Using the API console available via Site Admin, create a mutation

mutation UpsertEmptyBatchChange {
  upsertEmptyBatchChange(namespace: <namespace_id> name: "random-unknown-bc") {
    name
    id
    lastAppliedAt
    batchSpecs {
      totalCount
      nodes {
        id
        source
      }
    }
  }
}

• Ensure the name doesn't exist in your database.
• After running the mutation, the batch change should be automatically be created , on subsequent runs the mutation will not fail and will always return the existing batch change.