Handle batch-fetch requests in the replica

Add support for the `batch-fetch` endpoint in the
`PullReplicationFilter` class, which is used by the replica node in a
pull-replication setup.
Deserialise the json message into the appopriate class, and pass the
message for further processing to the relevant "action" class.

Bug: Issue 40015567
Change-Id: Id62a671f6436571846256d7dc4dac7c94416233c