Add consistent type hinting in fields.py

[veedubb]

Is your feature request related to a problem? Please describe.
This is not related to a specific problem, but is just to make the type hinting within the file consistent across both the LayerField and LayerFieldsContainer. The LayerField class is type hinted/annotated for non-dunder methods of the class, however the LayerFieldsContainer is missing similar type hints/annotations. Other non-private methods and functions throughout pyshark also offer similar type hinting to the LayerField class (XmlLayer).

Describe the solution you'd like
Similar type hinting/annotation in the LayerFieldsContainer class as the LayerField class.

Describe alternatives you've considered
Alternatively, type hinting/annotation could be removed, but based on the inclusion in other files it seems that type hinting/annotation is the correct application.

Additional context
None.

[veedubb]

Closed with #717