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.