[multi-radio] adding new public OT APIs for multi radio link config (#4440)

This commit adds new public OT definitions and API for radio link
configuration in `openthread/multi_radio.h`. Mainly the function
`otMultiRadioGetNeighborInfo()` is added which allows info about a
neighbor to be retrieved (e.g., which radio links are supported by the
neighbor and their preference value).

doc/ot_api_doc.h

@@ -79,6 +79,8 @@
  *
  * @defgroup api-message              Message
  *
+ * @defgroup api-multi-radio          Multi Radio Link
+ *
  * @defgroup api-thread               Thread
  *
  * @{