Gamepad improvements (#16222)

# Objective

Closes #16221.

## Solution

- Make `Gamepad` fields public and remove delegates / getters.
- Move `impl Into` to `Axis` methods (delegates for `Axis` used `impl
Into` to allow passing both `GamepadAxis` and `GamepadButton`).
- Improve docs.

## Testing

- I run tests.

Not sure if the migration guide is needed, since it's a feature from RC,
but I wrote it just in case.

---

## Migration Guide

- `Gamepad` fields are now public.
- Instead of using `Gamepad` delegates like `Gamepad::just_pressed`,
call these methods directly on the fields.

---------

Co-authored-by: Alice Cecile <[email protected]>

Author: 2 people

crates/bevy_input/src/axis.rs

@@ -45,16 +45,16 @@ where
     /// If the `input_device`:
     /// - was present before, the position data is updated, and the old value is returned.
     /// - wasn't present before, `None` is returned.
-    pub fn set(&mut self, input_device: T, position_data: f32) -> Option<f32> {
-        self.axis_data.insert(input_device, position_data)
+    pub fn set(&mut self, input_device: impl Into<T>, position_data: f32) -> Option<f32> {
+        self.axis_data.insert(input_device.into(), position_data)
     }
 
     /// Returns the position data of the provided `input_device`.
     ///
     /// This will be clamped between [`Axis::MIN`] and [`Axis::MAX`] inclusive.
-    pub fn get(&self, input_device: T) -> Option<f32> {
+    pub fn get(&self, input_device: impl Into<T>) -> Option<f32> {
         self.axis_data
-            .get(&input_device)
+            .get(&input_device.into())
             .copied()
             .map(|value| value.clamp(Self::MIN, Self::MAX))
     }
@@ -66,13 +66,13 @@ where
     /// Use for things like camera zoom, where you want devices like mouse wheels to be able to
     /// exceed the normal range. If being able to move faster on one input device
     /// than another would give an unfair advantage, you should likely use [`Axis::get`] instead.
-    pub fn get_unclamped(&self, input_device: T) -> Option<f32> {
-        self.axis_data.get(&input_device).copied()
+    pub fn get_unclamped(&self, input_device: impl Into<T>) -> Option<f32> {
+        self.axis_data.get(&input_device.into()).copied()
     }
 
     /// Removes the position data of the `input_device`, returning the position data if the input device was previously set.
-    pub fn remove(&mut self, input_device: T) -> Option<f32> {
-        self.axis_data.remove(&input_device)
+    pub fn remove(&mut self, input_device: impl Into<T>) -> Option<f32> {
+        self.axis_data.remove(&input_device.into())
     }
 
     /// Returns an iterator over all axes.