batches: support `createBatchSpecFromRaw` with existing batch change

Created by: courier-new

As another part of EM2.4, the mutation createBatchSpecFromRaw now takes as an additional optional argument a batchChange id, which, when provided, will be used to associate the batch spec that gets created to that existing batch change.

Corresponding changes to replaceBatchSpecInput can be found in the follow up PR #28827.