Add JSON serialization methods for Task and TaskResult classes

### Code of Conduct

- [x] I agree to follow Django's Code of Conduct

### Feature Description

Add JSON serialization methods for Task and TaskResult classes. Both ways, from and to JSON.

### Problem

The task framework already enforces call args and kwargs to be JSON serializable. However, there are not handy functions that can serialize and deserialize a TaskResult. This burden currently falls on the backends, which will always be awkward with the frozen dataclasses.

### Request or proposal

proposal

### Additional Details

_No response_

### Implementation Suggestions

@RealOrangeOne has already done the most homework here. :)

Start by locating the Task and TaskResult class definitions and reviewing how current backends handle their frozen dataclasses and JSON-serializable call arguments. Define the expected behavior in both directions, from and to JSON, including round-trip expectations; done means both classes have a consistent serialization and deserialization API that backends can use.

Written by the indexing model from the issue text.