Create infrastructure for testing external Dialect contributions

[schauder]

We are not in a position to support all the different databases out there.
But of course, we still want people to enable to use these databases.
Therefore we should create infrastructure that allows external contributors to run integration tests for their dialects, that give them good confidence that stuff actually works.

This infrastructure might also host a list of externally provided dialects.

[liubao68]

Sounds a good idea. Waiting more details about this plan.