Cover corner cases of Fields extension

[mcucchi9]

Related Issue(s):

• Fields extension: specifying both exclude and include for nested fields does not result in expected behavior #361
• Fields extension: specifying both exclude and include for the same field does not result in expected behavior #362

Description:

This PR:

• refactors logic in utils.py module, to facilitate testability;
• adds unit tests for pre-existing utils.py module functionalities;
• introduces logic to ensure filter extension is compliant with cases described in Fields extension: specifying both exclude and include for nested fields does not result in expected behavior #361 and Fields extension: specifying both exclude and include for the same field does not result in expected behavior #362, via the utils.clean_exclude function;
• adds unit tests for new funcionalities;
• fixes existing api tests and adds one more.

PR Checklist:

• pre-commit hooks pass locally
• Tests pass (run make test)
• Documentation has been updated to reflect changes, if applicable, and docs build successfully (run make docs)
• Changes are added to the CHANGELOG.