Use multipart/form data for query and its parameters

See https://clickhouse.com/docs/sql-reference/syntax#defining-and-using-query-parameters

Currently, the query itself goes into the body, and `Query::param` adds a parameter to the URL, which can cause issues with proxies.

Start with the Query::param entry point and the ClickHouse query-parameter documentation linked in the issue. Trace how the query currently enters the request body and parameters enter the URL; done means both are sent as multipart/form-data so parameters no longer rely on the URL.

Written by the indexing model from the issue text.