fix: fix custom config usage behaviour

[alohamora]

Description

This PR fixes the bug where foca did not register a custom config model which is passed in the input config if the api specs were not provided

Fixes #181

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
• I have updated the documentation (or documentation does not need to be updated)
• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (d4c1280) 100.00% compared to head (6489561) 100.00%.
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff            @@
##               dev      #183   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          978       984    +6     
=========================================
+ Hits           978       984    +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.