Add client support for NLPRP tabular schema

[RudolfCardinal]

We'd designed the specification, but not implemented it properly client-side. Now this matters!

[RudolfCardinal]

This change implements the client aspects of the tabular_schema option in the NLP Request Protocol (NLPRP); https://crateanon.readthedocs.io/en/latest/nlp/nlprp.html#list-processors.

There's an end-to-end test in the test suite that creates an SQLite database (requiring a few minor tweaks) and sets up a client-server pair. The client asks the server for its NLP definitions, then sets up a couple of database tables based on that. To be safe, remote table names aren't trusted, but if you map a remote table name to an empty definition, it'll use the remote tabular_schema structure.