feat: validate custom config

[uniqueg]

Description

• users can now provide a pydantic model class that is used to set custom configuration parameters via parameter custom_config_model to foca()
• parameters in section custom of the app configuration are used to populate/override the custom configuration, if available

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have not reduced the existing code coverage

[codecov]

Codecov Report

Merging #128 (10c74f3) into dev (944dc1e) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #128   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          670       689   +19     
=========================================
+ Hits           670       689   +19     

Impacted Files	Coverage Δ
foca/models/config.py	100.00% <ø> (ø)
foca/config/config_parser.py	100.00% <100.00%> (ø)
foca/foca.py	100.00% <100.00%> (ø)

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