Allow custom SQL connector

[Huy-Ngo]

Writing customized connectors to DBMS other than the default three might be desirable. Or, in my case, customize one of the existing three—I'd like to be able to inherit an SQL connector and modify it without changing the source code.

[Huy-Ngo]

I haven't investigated, but given that the SQL connectors are defined via an interface, I suppose this should be possible without updating the code at all, which makes this more of a documentation issue.