[Update] Change JsonConverter to allow dynamic type serialization

[eddy1937]

PR Title:

Allow json.JsonConverter to support dynamic type serialization for more flexibility

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PR Description:

Background / Problem:

When using swagger-dart-code-generator, the current implementation of json.JsonConverter forces type serialization to String, which limits flexibility when serializing custom types. For example, when attempting to serialize a DateTime object to a Unix timestamp (int), the code forces the output type to be String, leading to serialization errors.

Proposed Solution:

This PR modifies the generated code to use json.JsonConverter<T, dynamic> instead of json.JsonConverter<T, String>. This change allows for more flexibility in serializing various types, such as serializing DateTime to int (Unix timestamp) and other custom types.

Key Changes:

• Updated json.JsonConverter to support dynamic types instead of forcing the output to String.
• Ensured that the serialization logic can now handle different types, such as converting DateTime to a Unix timestamp (int).

Test Cases:

• I have tested this change locally and confirmed that it works as expected with custom serialization (e.g., serializing DateTime to int).
• No existing functionality has been broken by this update.

Related Issue:

This PR addresses issue #771.

[diegotori]

@Vovanella95 any progress on this?