Add metadata API to Tasks

### Code of Conduct

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

### Feature Description

A `TaskResult` should have `metadata` - a writable persistent `dict` (JSON-compatible) containing additional data about the task, but outside of its arguments.

Metadata should be settable as part of enqueueing, readable and modifiable during configuration, and persist.

### Problem

It's useful for a number of cases for a task result to have additional "metadata" attached to it, for example smuggling a telemetry trace id through to reconstruct trace timelines.

It's unlikely to enable many additional behaviours inside Django, but is mostly useful for the Tasks ecosystem.

### Request or proposal

proposal

### Additional Details

A similar API existed in `django-tasks`, but was removed to match what was upstreamed.

### Implementation Suggestions

The API:

- `my_task.using(metadata={"trace_id": 123}).enqueue(foo=1)` would enqueue the `my_task` task with metadata `{"trace_id": 123}` and arguments `foo=1`.
- Metadata could be retrieved using task context `context.metadata`
- If the `context.metadata` is modified, it is saved when a task finishes (successfully or otherwise)
- Metadata can be saved within a task using `context.save_metadata()` (or `.asave_metadata`)
- `metadata` is always a `dict`
- Metadata support is opt-in - backends must set the `supports_metadata` feature and implement `save_metadata` and/or `asave_metadata`
- Metadata keys prefixed with `_django` are reserve for internal implementation (eg for backend users). They may be stored, but are stripped out before being shown by the user, and are ignored during save.