Python vars in hybrid class

[freddieknets]

Description

• Allow python-only attributes in a HybridClass by defining them with _extra_fields_in_to_dict on the class (it was not possible to automatically recognise python-only attributes from _store_in_to_dict)
• Added a test which also demonstrates how the __init__ should be adapted correctly when using python-only attributes
• Make the to_dict method cascade into using the existing _to_json if available (e.g. for xofields that are Structs), which resolves deeper
• All tests pass, including in xtrack and xfields

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts