Clickhouse - Allow schema management for multiple databases within the same cluster.

[AndresElizondo]

The main thing keeping us from using dbmate to manage our ClickHouse migrations is that it only supports a single database at a time.
This limits the tables that are written to the schema.sql file to only the tables within the current database.

What I'm proposing is we add a connection parameter similar to search_path that allows for a list of comma-separated database names (like the parameter that exists for PostgreSQL).
This would look like: clickhouse://username:password@127.0.0.1:9000/database_name?search_path=myschema,public

We can then use this list to change the code at https://github.com/amacneil/dbmate/blob/main/pkg/driver/clickhouse/clickhouse.go#L177
From:
SHOW TABLES;
Into something like:

SELECT name FROM system.tables
WHERE
    NOT is_temporary
    AND database IN ('myschema','public')
;

The expected output in the schema.sql would then show all tables belonging to myschema and public databases.

Another way this could addressed would be to have a different dbmate folder for each database we want to manage migrations for and run migrations for each separately. (We're trying to move away from this since it can create dependencies between two different dbmate folders)

[AndresElizondo]

Created PR #515.