Add TextEnvelope class for saving and loading Witness and Transaction files

[KINGH242]

This pull request introduces a new TextEnvelope class to add JSON serialization and deserialization capabilities. It is added to the Transaction and VerificationKeyWitness classes, while adding new tests to ensure robustness. The TextEnvelope class can later be added to the various Certificates to allow them the same save and load functionality. Below are the key changes grouped by theme:

Core Enhancements

• Added a new TextEnvelope class in pycardano/serialization.py to enable JSON-based serialization and deserialization with fields like type, description, and cborHex. This class also includes methods for saving and loading JSON data to and from files.
• Updated Transaction and VerificationKeyWitness classes to inherit from TextEnvelope, enabling them to utilize its serialization features. Added custom KEY_TYPE and DESCRIPTION properties to these classes. [1] [2]

Utility and Validation Improvements

• Added a method is_empty to TransactionWitnessSet for checking whether the witness set is empty.
• Introduced equality (__eq__) and shallow primitive conversion (to_shallow_primitive) methods to VerificationKeyWitness to work with TextEnvelope.

Testing Enhancements

• Added tests for the save and load methods of Transaction and VerificationKeyWitness to validate JSON serialization and file handling. [1] [2]
• Expanded test coverage for DRep by introducing parameterized tests to handle various input scenarios, including edge cases and invalid inputs.

[codecov]

Codecov Report

Attention: Patch coverage is 97.59036% with 2 lines in your changes missing coverage. Please review.

Project coverage is 90.12%. Comparing base (7d63d0e) to head (04f1d48).

Files with missing lines	Patch %	Lines
pycardano/serialization.py	95.55%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #448      +/-   ##
==========================================
+ Coverage   88.86%   90.12%   +1.25%     
==========================================
  Files          33       33              
  Lines        4804     4880      +76     
  Branches      731      736       +5     
==========================================
+ Hits         4269     4398     +129     
+ Misses        374      310      -64     
- Partials      161      172      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cffls]

Thank you for raising this PR! Enabling classes to save to and load from json is amazing. I think it is possible to simplify the implementation by directly adding the json ability to CBORSerialzable. See the detailed comment below. Let me know if that makes sense to you. Thanks!

[cffls]

It might be a good idea to combine this directly with CBORSerializable, which would instantly enable all pycardano classes to save to and load from json. I don't think private fields, key_type and description, are necessary, because they usually don't change between instances. If we want to customize them for certain child class, the child class can always override the method json_key_type or json_description. Regarding payload, it becomes redundant once the logic is merged to CBORSerializable. The default implementation can directly use the class name as the key type, something like below:

class CBORSerializable:
    ....
    
    
    @property
    def json_key_type(self) -> str:
        return self.__class__.__name__

    @property
    def json_description(self) -> str:
        return self.__class__.__doc__
    
    def to_json(self) -> str:
        return json.dumps(
            {
                "type": self.json_key_type,
                "description": self.json_description,
                "cborHex": self.to_cbor_hex(),
            }
        )

Basic usage

Custom key type