Make SqlAlchemyConnector work with database schemas (backport to 0.7)

[soininen]

It is now possible to pass database schema to SqlAlchemyConnector.connect_to_source()

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
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (f5d5209) 75.35% compared to head (06acf0e) 75.31%.

Files	Patch %	Lines
...edb_api/spine_io/importers/sqlalchemy_connector.py	40.00%	6 Missing :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #324 +/- ## ========================================== - Coverage 75.35% 75.31% -0.05% ========================================== Files 76 76 Lines 10978 10984 +6 Branches 2390 2391 +1 ========================================== Hits 8273 8273 - Misses 2267 2273 +6 Partials 438 438 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.