Add support for DSR, DCD, and RI to Adafruit_TinyUSB_Arduino CDC profile

[SmittyHalibut]

Is your feature request related to a problem? Please describe.
I need to emulate an FT231 serial chip, the hardware control signals. I use CTS, DSR, and DCD to trigger events on the Host. (Ham Radio software, decades old, triggers PTT, and morse code paddles, based on these flow control signals.)

Describe the solution you'd like
I need a way to trigger these signals up-stream to the USB Host. We have Serial.dtr() and Serial.rts() for reading control signals from the Host. I want Serial.cts(), Serial.dsr(), and Serial.dcd() too, to send those signals to the host. Similarly to how the Python serial library works. (And probably Serial.ri() since I'm there, might as well complete the whole set.)

Describe alternatives you've considered
Some Ham Radio software is moving towards MIDI for triggering these events instead, which I have already implemented. But I want to remain backward compatible with old/existing software.

I could continue to use an FT231 chip, but I can't add MIDI to the FT231. I would need a USB Hub, and both my microcontroller (RP2040, but I don't think that's relevant here) and the FT231 chip. I'd rather have both CDC and MIDI endpoints on the same USB device.

Additional context
Changing the host software that's consuming these signals is a non-starter. Or rather, it's already happening (moving to MIDI), but it's happening VERY SLOWLY. I want to remain backward compatible where possible.

I intend to attempt this code change myself. See SmittyHalibut/Adafruit_TinyUSB_Arduino

Update: See #493

Update: USB CDC ACM spec doesn't include CTS. :-( But I've implemented all the rest.

[SmittyHalibut]

I've officially submitted #493 for consideration.