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

• Documentation (also in Toolbox repo) is up-to-date
• Release notes in Toolbox repo have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black
• 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 ⚠️
spine_items/importer/importer.py	0.00%	3 Missing ⚠️
spine_items/importer/connection_manager.py	83.33%	1 Missing and 1 partial ⚠️
spine_items/importer/importer_factory.py	0.00%	2 Missing ⚠️
...ine_items/importer/widgets/import_editor_window.py	60.00%	2 Missing ⚠️
spine_items/exporter/widgets/export_list_item.py	0.00%	1 Missing ⚠️
spine_items/importer/executable_item.py	83.33%	0 Missing and 1 partial ⚠️
spine_items/widgets.py	91.66%	1 Missing ⚠️

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Additional details and impacted files

@@             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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