Add CDC+UAC2 composite device example for Pico

Signed-off-by: Dhiru Kholia <dhiru.kholia@gmail.com>

examples/device/cdc_uac2/README.md

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+#### Composite CDC + UAC2 on Pico
+
+This example provides a composite CDC + UAC2 device on top of a Raspberry Pi
+Pico board.
+
+
+#### Use Cases
+
+- The CDC + UAC2 composite device happens to be important, especially in the
+  amateur radio community.
+
+  Modern radios (`rigs`) like Icom IC-7300 + IC-705 expose a sound card and a
+  serial device (`composite device`) to the computer over a single USB cable.
+  This allows for Audio I/O and CAT control over a single USB cable which is
+  very convenient.
+
+  By including and maintaining this example in TinyUSB repository, we enable
+  the amateur radio community to build (`homebrew`) radios with similar
+  functionality as the (expensive) commercial rigs.
+
+  This PR is important in bridging this specific gap between the commercial
+  rigs and homebrew equipment.
+
+- https://digirig.net/digirig-mobile-rev-1-9/ is a digital interface for
+  interfacing radios (that lack an inbuilt digital interface) with computers.
+  Digirig Mobile works brilliantly (is OSS!) and is a big improvement over
+  traditional digital interfaces (like the SignaLink USB Interface). By using a
+  Raspberry Pi Pico powered CDC + UAC2 composite device, we can simplify the
+  Digirig Mobile schematic, drastically reduce the manufacturing cost, and
+  (again) enable the homebrewers community to homebrew a modern digital interface
+  with ease themselves.
+
+
+#### Build Steps
+
+```
+cd examples/device/cdc_uac2
+
+export PICO_SDK_PATH=$HOME/pico-sdk
+
+cmake -DFAMILY=rp2040 pico .
+
+cmake -DFAMILY=rp2040 -DCMAKE_BUILD_TYPE=Debug # use this for debugging
+
+make BOARD=raspberry_pi_pico all
+```
+
+
+#### Development Notes
+
+Please try to keep this code synchronized with the `uac2_headset` example
+included in this repository.