[Feat] Add Beacon.derivedStream along with tests

[jinyus]

Description

Specialized DerivedBeacon that subscribes to the stream returned from its callback and updates its value based on the emitted values. When a dependency changes, the beacon will unsubscribe from the old stream and subscribe to the new one.

Example:

final userID = Beacon.writable<int>(18235);
final profileBeacon = Beacon.derivedStream(() {
 return getProfileStreamFromUID(userID.value);
});

• [x] ✨ New feature (non-breaking change which adds functionality)
• [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
• [ ] ❌ Breaking change (fix or feature that would cause existing functionality to change)
• [ ] 🧹 Code refactor
• [ ] ✅ Build configuration change
• [ ] 📝 Documentation
• [ ] 🗑️ Chore

[jinyus]

Test used to validate behaviour:

@lukepighetti let me know if there's something that this test doesn't cover.

  test('should unsub from old stream when dependency changes', () async {
    final controller = StreamController<int>.broadcast();

    final counter = Beacon.writable(5);

    // should increment when dependency changes
    var unsubs = 0;
    var listens = 0;

    controller.onCancel = () => unsubs++;

    controller.onListen = () {
      listens++;
      addItems(controller, counter.value);
    };

    final beacon = Beacon.derivedStream(() {
      counter.value;
      return controller.stream;
    });

    expect(listens, 1);

    counter.increment(); // dep changed, should unsub from old stream

    expect(unsubs, 1);
    expect(listens, 2);

    counter.increment();

    expect(unsubs, 2); // dep changed, should unsub from old stream
    expect(listens, 3);

    beacon.dispose(); // should unsub when disposed

    expect(unsubs, 3);
    expect(listens, 3);
  });

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (bde8f95) 100.00% compared to head (d5ad1b8) 100.00%. Report is 88 commits behind head on main.

Additional details and impacted files

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[lukepighetti]

LGTM!