fix: forcings

[flyIchtus]

Description

Attempting to fix the behaviour encountered in the below-mentioned issue:

1. Set the correct input type for the dates in forcing FunctionResult and forward only list of dates to earthkit-data backend
2. Set args kwargs into FunctionAction source (otherwise deacorators such as legacy_source don't know them).

This code fixes the mentioned issues, but might break other components (although I doubt it).

Type of Change

• 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

Issue Number

Solving bugs arising in #262

Code Compatibility

• I have performed a self-review of my code

Code Performance and Testing

• I have added tests that prove my fix is effective or that my feature works
• I ran the complete Pytest test suite locally, and they pass

Dependencies

• I have ensured that the code is still pip-installable after the changes and runs
• I have tested that new dependencies themselves are pip-installable.

Documentation

• My code follows the style guidelines of this project
• I have updated the documentation and docstrings to reflect the changes
• I have added comments to my code, particularly in hard-to-understand areas

[b8raoult]

The bug needed some lower level fixes. Here: #269