Add column case option to IMPORT FOREIGN SCHEMA

Since 54342bb, pg_clickhouse has been using `quote_identifier()` for all tables and columns imported from ClickHouse. As of #117 (but really for some time), one has been able to specify a custom column name in `CREATE FOREIGN TABLE` column options.

Add an option for [IMPORT FOREIGN SCHEMA](https://www.postgresql.org/docs/current/sql-importforeignschema.html) to tell the importer to prefer lowercase names and only fall back on quoting if there is a conflict with an earlier name.