[M.D.Sqlite] improve support for GetSchema()

ADO.NET has a weakly-typed introspection API, where users can call DbConnection.GetSchema() and get information about the database (and the driver). This API is lacking in various ways - it's weakly typed and support varies across providers - but it's the only semi-standard thing, and there seems to be at least some sort of common denominator that's implemented across provider. Microsoft.Data.Sqlite currently doesn't support this - we can consider implementing a more support here to align with other ADO.NET providers.

/cc @bricelam (I know you hate GetSchema() - as do I - but that's what we have...)