[posix-app] get RCP (NCP in radio mode) version (#3140)

This commit contains the following changes:

- It adds `otPlatRadioGetVersionString()` as an optional radio
  platform API. If the radio platform does not implement this
  function, OpenThread core provides a default weak implementation
  of this function which return the OpenThread version.

- A public OpenThread API `otGetRadioVersionString()` is added which
  provides the radio version string.

- POSIX App is updated to get the version from its RCP and provide
  `otPlatRadioGetVersionString()`

- A new spinel capability `SPINEL_CAP_POSIX_APP` is added which
  corresponds to `ENABLE_POSIX_APP` configuration.

- A new spinel property `SPINEL_PROP_RCP_VERSION` (only available
  in POSIX_APP) is added which gets the RCP version.

tests/toranj/openthread-core-toranj-config.h

@@ -40,7 +40,13 @@
  * The platform-specific string to insert into the OpenThread version string.
  *
  */
+#if OPENTHREAD_RADIO
+#define OPENTHREAD_CONFIG_PLATFORM_INFO                         "POSIX-RCP-toranj"
+#elif OPENTHREAD_ENABLE_POSIX_APP
+#define OPENTHREAD_CONFIG_PLATFORM_INFO                         "POSIX-App-toranj"
+#else
 #define OPENTHREAD_CONFIG_PLATFORM_INFO                         "POSIX-toranj"
+#endif
 
 /**
  * @def OPENTHREAD_CONFIG_NUM_MESSAGE_BUFFERS