Add support for database schemas into Data connection, Importer (backport to 0.7)

[soininen]

It is now possible to specify a database schema when adding a db URL reference to Data connection. Additionally, Importer now works with the schemas.

Re spine-tools/Spine-Toolbox#2329

Checklist before merging

• [x] Documentation (also in Toolbox repo) is up-to-date
• [x] Release notes in Toolbox repo have been updated
• [x] Unit tests have been added/updated accordingly
• [x] Code has been formatted by black
• [ ] Unit tests pass