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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

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

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