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