Ergonomic observer queries: SystemInput-taking SystemParam edition

[ItsDoot]

Objective

• Closes Provide easy access to components of targeted entity in observers #15653

Solution

SystemParams now take the SystemInput by reference when fetching the data for the system:

1. Add trait SystemParamFetch<I: SystemInput>: SystemParam, a new super trait of SystemParam.
2. Move SystemParam::get_param to SystemParamFetch, and add a new argument to it that accepts a reference to the SystemInput.
3. Implement SystemParamFetch for any I: SystemInput for all current SystemParam implementations, and move the get_param implementation over.
4. Add functions to SystemState to work with non-() system inputs.
5. Update FunctionSystem trait machinery from P: SystemParam to P: SystemParamFetch<System::In>.
6. Add TargetEntity trait and Target<D, F> query type. The latter is similar to Single<D, F>, but fetches the query data for the target entity rather than the unique entity that matches the query.
   a. Target<D, F> is bounded to only work with SystemInputs that take a type that implements TargetEntity.

Benefits of this approach:

• Compile-time checked: if you use Target<D, F> in a normal scheduled system (which has no input), you get an error when trying to register the system via add_systems.
• No hidden state: The target Entity used by Target<D, F> is the same data available via On<E>.
• Extensible: supports SystemParam fetching based on any SystemInput
  • Which means you could implement an OriginalTarget<D, F> for propagating events, for example.

Drawbacks of this approach:

• Type complexity: while custom SystemParam implementations are already in advanced territory, complicating the API surface further does have an impact.
  • This can be (and has partly been) remediated with documentation on good practices.

Testing

Added a test for Target<D, F>.

---

Showcase

Ergonomic target entity queries are here!

fn my_observer(on: On<Interact>, button: Target<&Name, With<Button>>) {
    // The observer's function only executes if the Target query matches the target entity
    let name = *button;
    println!("Button {name} was interacted with");
}

app.add_observer(my_observer);