[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.

include/openthread/instance.h

@@ -379,13 +379,23 @@ otLogLevel otGetDynamicLogLevel(otInstance *aInstance);
 otError otSetDynamicLogLevel(otInstance *aInstance, otLogLevel aLogLevel);
 
 /**
- * Get the OpenThread version string.
+ * This function gets the OpenThread version string.
  *
  * @returns A pointer to the OpenThread version.
  *
  */
 OTAPI const char *OTCALL otGetVersionString(void);
 
+/**
+ * This function gets the OpenThread radio version string.
+ *
+ * @param[in]  aInstance A pointer to an OpenThread instance.
+ *
+ * @returns A pointer to the OpenThread radio version.
+ *
+ */
+const char *otGetRadioVersionString(otInstance *aInstance);
+
 /**
  * @}
  *

include/openthread/platform/radio.h

@@ -634,6 +634,19 @@ int8_t otPlatRadioGetReceiveSensitivity(otInstance *aInstance);
  */
 extern void otPlatRadioFrameUpdated(otInstance *aInstance, otRadioFrame *aFrame);
 
+/**
+ * Get the radio version string.
+ *
+ * This is an optional radio driver platform function. If not provided by platform radio driver, OpenThread uses
+ * the OpenThread version instead (@sa otGetVersionString()).
+ *
+ * @param[in]  aInstance   The OpenThread instance structure.
+ *
+ * @returns A pointer to the OpenThread radio version.
+ *
+ */
+const char *otPlatRadioGetVersionString(otInstance *aInstance);
+
 /**
  * @}
  *