Serialization and deserialization of structs and other fields

[satvikpendem]

Is your feature request related to a problem? Please describe.
I want to use JSON serialization in FRB because I have JSON data I generate on the Rust side and then need to show that in the UI on the Flutter side.

Describe the solution you'd like
There are a few solutions I thought of:

• Since we already use freezed, we could make freezed be able to generate to and from JSON methods as they are already supported in freezed itself. This is basically Let the generated freezed class be able to have fromJson/toJson #1726.
• On the Rust side, if we annotate a struct with #[derive(Serialize, Deserialize), perhaps it could be generated automatically on the Dart side just as serde does on the Rust side. The Dart side could even do so via the above freezed way.

Describe alternatives you've considered
Changing the generated code to use the freezed JSON methods or writing it manually in the Dart or Rust code. This can be brittle though, especially changing generated code.

Additional context
NA

[fzyzcjy]

I guess the first solution looks good - we only need to add several lines. Feel free to PR for this! Alternatively, I may work on it in the next batch (since this is a new feature instead of a bug).

[satvikpendem]

Just made a PR: #2470, had a question about tests there