usb: builtin handling of interface alternate settings

The stack reads its own descriptors to figure out which endpoints
are used in which alt settings, and enables/disables them as needed.

The ControlHandler has a callback so it can get notified of alternate
setting changes, which is purely informative (it doesn't have to do anything).

embassy-usb/src/types.rs

@@ -106,7 +106,7 @@ pub struct EndpointInfo {
 /// A handle for a USB interface that contains its number.
 #[derive(Copy, Clone, Eq, PartialEq)]
 #[cfg_attr(feature = "defmt", derive(defmt::Format))]
-pub struct InterfaceNumber(u8);
+pub struct InterfaceNumber(pub(crate) u8);
 
 impl InterfaceNumber {
     pub(crate) fn new(index: u8) -> InterfaceNumber {