feat: add db_engine_specs/risingwave.py to support time grain

[BowenXiao1999]

Hi, represents RisingWave. Want to add RisingWave into Superset's codebase for time grain support. RisingWave is a postgresql-compatitable database with sqlalchemy dialect. The driver is in here: https://github.com/risingwavelabs/sqlalchemy-risingwave.

I may need to add a db_engine_specs/risingwave.py. For the first version, I think highly alike db_engine_specs/cockroachdb.py.
I tried locally, only after add this file, the time grain option is going to work. (PS: Although we support postgresql protocol, the connection URI string is risingwave://.... instead of postgresql://...)

This issue is created before the PR. Glad to see your comments! Thanks.