Allow importing metadata

[PiispaH]

Adds import mappings for entity- and parameter value metadata so that they can be imported.

Fixes spine-tools/Spine-Toolbox#2831

Checklist before merging

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 35.71429% with 18 lines in your changes missing coverage. Please review.

Project coverage is 78.17%. Comparing base (9fca258) to head (f5462e1).

Files	Patch %	Lines
spinedb_api/import_mapping/import_mapping.py	12.50%	14 Missing :warning:
spinedb_api/import_mapping/generator.py	66.66%	2 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #419 +/- ## ========================================== - Coverage 78.28% 78.17% -0.11% ========================================== Files 72 72 Lines 9517 9543 +26 Branches 2034 2038 +4 ========================================== + Hits 7450 7460 +10 - Misses 1731 1745 +14 - Partials 336 338 +2 ```

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