Introduce `batch-fetch` REST API Endpoint

Similar to the `batch-apply-object` endpoint, there are the same
benefits if we batch the refs into one fetch call.

Whether to use the batching endpoint is configurable (false by default)
with the existing behaviour preserved for backwards compatibility.

Bug: Issue 40015567
Change-Id: Idb83e5cfcc4e8aae33c28f906518cad56ae9fc13
9 files changed