[spinel] add support for multiple spinel interfaces (#9360)

This feature allows the RCP to support multiple host stacks on different PANs
by making use of the spinel Interface ID.

Created unit tests for testing multipan feature with multiple ot-instance
support.

Based on Si-Labs PR #8914 by @parag-silabs, but a little different approach.
Instead of handling everything by a single sub-mac instance, multiple
OpenThread instances are created on RCP side that map to different IID.
Thanks to this there are separate data kept for each interface. Platform
is able to determine interface by ot instance pointer passed as an argument
to most of the API functions.
Tx/scan queue was removed as it is possible to request transmission in
parallel, it is up to the platform to decide if it should fail or queue
second tx or it has two radios available.

NOTE:
Platform needs to provide different otRadioFrame of each instance and
the processing needs to take into account the instance being used.

Signed-off-by: Marek Porwisz <[email protected]>

doc/ot_api_doc.h

@@ -176,6 +176,7 @@
  * @defgroup plat-memory              Memory
  * @defgroup plat-messagepool         Message Pool
  * @defgroup plat-misc                Miscellaneous
+ * @defgroup plat-multipan            Multipan
  * @defgroup plat-otns                Network Simulator
  * @defgroup plat-radio               Radio
  * @defgroup plat-settings            Settings