Add support for the `attach` command

[tantaman]

We don't support attaching dbs at the moment. Unclear if it should even be supported but it does make testing and exploration simpler.

[tantaman]

this makes it somewhat problematic: https://www.sqlite.org/vtab.html#eponymous_virtual_tables

Eponymous virtual tables exist in the "main" schema only, so they will not work if prefixed with a different schema name.

would require quite a bit of re-work to crsql_changes