rollupJoins on multiple DBs of different types

**Is your feature request related to a problem? Please describe.**
When performing a `rollupJoin` on two different databases, this is not possible supported with different DB types, such as Mongo and Snowflake. In other cases it can still work but perform slowly.

**Describe the solution you'd like**
Cube Store can do a `rollupJoin` on databases from two different DB types.

**Describe alternatives you've considered**
Consolidating all my data on one DB, instead of using multiple data sources. This requires migration/ETL work on my end, though. I could also set up external pre-aggregations, but this adds complexity and then I can't use the optimizations of Cube Store here.

**Additional context**
We also discussed this in Slack per https://cube-js.slack.com/archives/CC0403RRR/p1630176403090300