[Feature request] Implement the multipan stuffs from Silicon Labs into ot-br-posix

[caipiblack]

We are using EFR32 dongle with multipan firmware (this is a solution from Silicon Labs to use the same dongle for Zigbee, Thread and Bluetooth communication)

Currently we have to use ot-br-posix version from gecko_sdk because they made some modifications in Openthread to support the multipan stuff that is not yet in the official repository

In their implementation, openthread connects to a daemon called cpcd:

So maybe their modifications are around the radio-url and connection stuff

It should be very good, to merge their stuff in the official ot-br-posix repository, so that the user will be able to use the last versions of openthread instead of versions fixed in the gecko_sdk. gecko sdk

Application note:

https://www.silabs.com/documents/public/application-notes/an1333-concurrent-protocols-with-802-15-4-rcp.pdf

[lmnotran]

Hi @caipiblack,

We're actively working on upstreaming the multipan RCP changes. Stay tuned!

https://github.com/openthread/openthread/pull/8914

[lmnotran]

CC: @parag-silabs @romacdon @bertoldi-silabs @suveshpratapa