Add a required_callback to inject user defined requirement checks

[VolkerChristian]

In some situations the standard requirement checks are not sufficient. In such cases the user can provide a required_callback doing additional requirement checks. This callback is executed after all standard requirement checks.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.98%. Comparing base (e4ee3af) to head (3952a05).
⚠️ Report is 126 commits behind head on main.

Files with missing lines	Patch %	Lines
include/CLI/impl/App_inl.hpp	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #1213      +/-   ##
===========================================
- Coverage   100.00%   99.98%   -0.02%     
===========================================
  Files           17       19       +2     
  Lines         4546     5247     +701     
  Branches         0     1067    +1067     
===========================================
+ Hits          4546     5246     +700     
- Misses           0        1       +1     

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[phlptp]

will need a few test cases and some documentation. Do you have an example use case that is driving this change?