rollupJoins on multiple DBs of different types

[ryanpei]

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