Add support for database schemas into Data connection, Importer

[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
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (2d2d9dd) 60.14% compared to head (73bdbca) 60.06%.

Files	Patch %	Lines
spine_items/importer/do_work.py	29.41%	12 Missing :warning:
spine_items/importer/importer.py	0.00%	3 Missing :warning:
spine_items/importer/connection_manager.py	83.33%	1 Missing and 1 partial :warning:
spine_items/importer/importer_factory.py	0.00%	2 Missing :warning:
...ine_items/importer/widgets/import_editor_window.py	60.00%	2 Missing :warning:
spine_items/exporter/widgets/export_list_item.py	0.00%	1 Missing :warning:
spine_items/importer/executable_item.py	83.33%	0 Missing and 1 partial :warning:
spine_items/widgets.py	91.66%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #171 +/- ## =========================================== - Coverage 60.14% 60.06% -0.08% =========================================== Files 194 194 Lines 17650 17615 -35 Branches 2872 2872 =========================================== - Hits 10616 10581 -35 + Misses 6469 6468 -1 - Partials 565 566 +1 ```

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