Provide column info

[JanJakes]

WPDB exposes a get_col_info() method that retrieves column metadata from the last query using mysqli_num_fields and mysqli_fetch_field.

PDO statement provides analogous methods columnCount and getColumnMeta, but in the case of the SQLite driver, we may sometimes execute multiple queries for a single MySQL query, so the column info may need to be reconstructed more manually in some cases.

It may also happen that the returned column info needs to be "translated" from SQLite data to MySQL data using the information stored in the information_schema tables.

This also relates to Automattic/sqlite-database-integration#7 regarding how the column info should be exposed.