Add support for conversion of in memory tables to protobuf

[timsaucer]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

This may need to be performed upstream (or at least verified) but currently when testing on #892 I discovered that I could convert to/from protobuf for plans that use csv tables, but I cannot do so for those created from a record batch in memory.

Describe the solution you'd like

These tables should be supported for conversion.

Describe alternatives you've considered

None

[aditanase]

What would be the expectation in that case? A MemTable resides on your heap, while CSV is potentially on disk. Would you expect that the data would be serialized as part of the plan? How would the execution work on the other side?

I've seen some of this done in the substrait project:
https://substrait.io/relations/logical_relations/#virtual-table

I would imagine that one could write an extension codec that serializes the arrow table as well, but it would be extra work, not sure of the use case though.

[timsaucer]

@aditanase makes a very good point. Looking back on this a year later and it makes sense to me that we do not need to support these tables.