feat: add ``soft delete`` mixin

[Kumzy]

trafficstars

Creating a mixin for soft delete.

Do you think we need more in the Mixin?

Missing tests + repository changes + documentation

[codecov-commenter]

Codecov Report

:x: Patch coverage is 0% with 14 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 80.17%. Comparing base (c08ef25) to head (69eefbd). :warning: Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
advanced_alchemy/mixins/softdelete.py	0.00%	14 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #337      +/-   ##
==========================================
- Coverage   80.32%   80.17%   -0.16%     
==========================================
  Files          87       88       +1     
  Lines        6405     6447      +42     
  Branches      828      838      +10     
==========================================
+ Hits         5145     5169      +24     
- Misses       1000     1014      +14     
- Partials      260      264       +4     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.