Introduce API to backup DBs

[manuelma]

Introduce API to support https://github.com/spine-tools/Spine-Toolbox/issues/2521

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[bot]]

Codecov Report

Attention: 28 lines in your changes are missing coverage. Please review.

Comparison is base (aaa889d) 77.58% compared to head (6b48807) 77.36%.

Files	Patch %	Lines
spinedb_api/db_mapping.py	10.00%	26 Missing and 1 partial :warning:
spinedb_api/helpers.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #366 +/- ## =========================================== - Coverage 77.58% 77.36% -0.22% =========================================== Files 75 75 Lines 9372 9401 +29 Branches 1984 1988 +4 =========================================== + Hits 7271 7273 +2 - Misses 1774 1800 +26 - Partials 327 328 +1 ```

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