Add library for reset interface

[will-v-pi]

This separates the stdio_usb reset interface out into a separate library, which can be used by applications which use TinyUSB directly.

No change in behaviour when using the typical stdio_usb (eg in the hello_usb example).

Keeps the PICO_STDIO_USB_ prefix on the config defines to maintain compatibility, and also maintains compatibility with picotool.

Instructions to use this are added to the header file.

See implementation in debugprobe and the dev_multi_cdc example

[kilograham]

not sure resetd is a sufficient/good prefix for the public API

[kilograham]

note also, on this and the below, this is peculiar to TinyUSB - as we think about CherryUSB etc, we (i haven't looked in detail) should consider what if any of this is exposed (and whether we should expose them again in a separate header - e.g. one for TinyUSB one for CherryUSB)

[will-v-pi]

I will rename them to pico_usb_reset_interface_...

Note that these functions are not supposed to be exposed as a public API - the only thing that should be used publicly is the _resetd_driver struct, I will try to refactor it so that's the case

[will-v-pi]

I made a separate TinyUSB header, and made it so this should be compatible with the CherryUSB PR (as that only uses the usb_reset_interface.h header)

I couldn't refactor the functions out of the header, because I think the pico_usb_reset_interface_driver has to be defined in the header as static const in order for the debugprobe inclusion to compile, and therefore the functions must be in the header too

[kilograham]

I will rename them to pico_usb_reset_interface_...

Note that these functions are not supposed to be exposed as a public API - the only thing that should be used publicly is the _resetd_driver struct, I will try to refactor it so that's the case

note pico_/hardware_ is not included in function names; e.g. dma_foo