Add support for batch object info into ExecutionEnvironments

[alexbarcelo]

Add a new RPC in ExecutionEnvironments for retrieving batch information about the objects.

The current use case for this call is to enable a performant way to retrieve extra information, to be used by the split, and use a single RPC call per ExecutionEnvironment (instead of doing a RPC per object).

This is useful because the split will need information about a bunch of objects (e.g. the split needs to know if the objects are in-memory or have been evicted into disk) and the split can use the object hint to aggregate all the objects into a single batch call to the ExecutionEnvironment.

The proof-of-concept implementation will allow to ask for "is_in_memory", but further metadata may be provided in the future (e.g. replica stuff, versions, memory tier, placement, whtaever).