Add reset interface to multi CDC example

[will-v-pi]

This depends on raspberrypi/pico-sdk#2629 which has now been merged

This adds a reset interface to the dev_multi_cdc example, along with other improvements to the responsiveness and structure

[mtfurlan]

This allows picotool to put the board in BOOTSEL mode, but the serial numbers of the example and the bootloader don't match so picotool can't find it.

$ picotool info --vid 0xcafe --pid 0x4102 -f
Tracking device serial number DF609C8067295728 for reboot
The device was asked to reboot into BOOTSEL mode so the command can be executed.
Waiting for device to reboot.........

Despite the reboot attempt, no accessible RP-series devices in BOOTSEL mode were found found with serial number DF609C8067295728. It is possible the device is not responding, and will have to be manually entered into BOOTSEL mode.

[  +0.252727] usb 9-1.4: new full-speed USB device number 104 using xhci-pci-renesas
[  +8.606144] usb 9-1.4: New USB device found, idVendor=cafe, idProduct=4102, bcdDevice= 1.00
[  +0.000020] usb 9-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[  +0.000010] usb 9-1.4: Product: Pico (2)
[  +0.000008] usb 9-1.4: Manufacturer: Raspberry Pi
[  +0.000007] usb 9-1.4: SerialNumber: DF609C8067295728
[  +0.025451] cdc_acm 9-1.4:1.0: ttyACM1: USB ACM device
[  +0.003981] cdc_acm 9-1.4:1.2: ttyACM2: USB ACM device
[ +12.616681] usb 9-1.4: USB disconnect, device number 104
[  +0.256015] usb 9-1.4: new full-speed USB device number 105 using xhci-pci-renesas
[  +0.141344] usb 9-1.4: New USB device found, idVendor=2e8a, idProduct=0003, bcdDevice= 1.00
[  +0.000022] usb 9-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[  +0.000010] usb 9-1.4: Product: RP2 Boot
[  +0.000008] usb 9-1.4: Manufacturer: Raspberry Pi
[  +0.000007] usb 9-1.4: SerialNumber: E0C912952D54

[kilograham]

This seems like a bad idea - there is no locking on the called tud_task(), so may interfere with the tasks in the loop below - perhaps the example should just use the task from the loop as before, but do something less silly (i.e. use a state machine rather than a sleep) in custom_cdc_task() - not really sure that custom_cdc_task is very interesting as is anyway