Implement table/column reflection

[lidavidm]

See lidavidm/arrow#9

Similar to AdbcConnectionGetTables, there should be a way to query the columns available in a table with their names, types, NULL-ness etc.

I was basically modeling this off of Flight SQL: https://github.com/apache/arrow/blob/master/format/FlightSql.proto

In this case GetTables has a parameter to optionally fetch the Arrow schema of the tables as well. And then SQL-specific type info is encoded as column metadata.

I recall we talked about using something like MSSQL information_schema instead. This was also debated for Flight SQL. We decided against it since databases don't necessarily implement this, or implement slightly different things, and there's no easy way for a client to figure this out. Also, then you're dependent on knowing the database's SQL dialect in order to introspect it. While explicit calls like GetTables are tedious and less flexible, they don't require so much bootstrapping. (This is important for Flight SQL which does not want to have to implement client-side drivers; a Flight SQL client should be compatible with any database implementing the protocol. ADBC doesn't necessarily have the same constraint, though.)

GetTables needs to also (optionally) return the schema. Or we could have a separate method that models the schema explicitly? The difference would be that we could explicitly model the SQL-database metadata (which Flight SQL stuffs into Arrow field metadata)

[lidavidm]

Also see lidavidm/arrow#6 where we need to clarify the signature for GetTables