Add Session Settings to `FOREIGN SERVER`s and `USER MAPPING`s

[theory]

Record any ClickHouse Session Settings passed via the options to CREATE SERVER and CREATE USER MAPPING and merge them with settings from the pg_clickhouse.session_settings GUC. Precedence is:

1. Settings in queries
2. Settings for a user mapping
3. Settings from a server
4. Settings in the GUC

It probably doesn't make sense to support settings in foreign tables, since they may or may not be appropriate for a given query, but evaluate it and make a final call.