Make SqlAlchemyConnector work with database schemas

[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 (cc90681) 76.97% compared to head (9767aaa) 76.85%.

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

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #315 +/- ## =========================================== - Coverage 76.97% 76.85% -0.12% =========================================== Files 74 74 Lines 9181 9187 +6 Branches 1949 1950 +1 =========================================== - Hits 7067 7061 -6 - Misses 1780 1790 +10 - Partials 334 336 +2 ```

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