support for "prewhere"

[Fleapse]

hey, clickhouse lang has "prewhere" condition is there any way to set this condition using django-clickhouse-backend? and if there is no such method, is it possible to plan to add it for future releases?

[jayvynl]

To support this:

1. Add prewhere attribute to clickhouse_backend.models.sql.query.Query as a container for prewhere clauses.
2. Add prewhere method to clickhouse_backend.models.query.QuerySet which update clickhouse_backend.models.sql.query.Query.prewhere.
3. Generate prewhere SQL in clickhouse_backend.models.sql.query.compiler.*Compiler.as_sql.

This may take some time to implement.