Gamepads cannot be easily manually registered or deregistered

## What problem does this solve or what need does it fill?

Users want to manually register and deregister gamepads as part of tests to verify that gamepad inputs are working correctly.

The critical methods / fields on the [Gamepads](https://dev-docs.bevyengine.org/bevy/input/gamepad/struct.Gamepads.html) struct are private however, which makes doing so rather convoluted.

## What solution would you like?

Make the `register` and `deregister` [methods](https://github.com/bevyengine/bevy/blob/61a3494a065673314c6f59374ca9f52a7566ceea/crates/bevy_input/src/gamepad.rs#L38) on `Gamepads` public.

## What alternative(s) have you considered?

We could expose the underlying HashSet directly, but that flavor of API is somewhat harder to read.

Users can already do this with a horrible workaround where they manually run the public `gamepad_connection_system`.