esp-adf-libs includes a hidden copy of libopus

[hossbeast]

I've noticed that esp-adf-libs includes a bundled copy of libopus, from https://github.com/xiph/opus. I described my findings here - XasWorks/esp-libopus#3

My request is that esp-adf-libs 1) make it clear that libopus is included with esp-adf-libs, including precise version information, and 2) vend the libopus header files from whatever version it was built from.

Opus is great, and folks have a need for it to be built for esp32. Make it obvious that it's there and make it easy to use - that can only help the users of esp-adf-libs.

[jason-mao]

@hossbeast The copyrights notice on https://docs.espressif.com/projects/esp-adf/en/latest/COPYRIGHT.html. The version of opus is v1.2.1, which will be update on esp-adf-libs.