[posix-app] check RCP capabilities and add support for stream log (#3215)

This commit contains the following changes:

- It adds code to `RadioSpinel::Init()` to get and check the RCP
  capabilities (`SPINEL_PROP_CAPS`) ensuring RCP supports radio/raw
  mode (`SPINEL_CAP_MAC_RAW`).

- It adds support for handling stream log `SPINEL_PROP_STREAM_LOG`
  property from RCP.

- It moves the existing code checking spinel version and radio
  capabilities of RCP into helper methods `CheckSpinelVersion()`
  and `CheckRadioCapabilities()`.

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

@@ -170,6 +170,14 @@
  */
 #define OPENTHREAD_CONFIG_LOG_SUFFIX                            ""
 
+/**
+ * @def OPENTHREAD_CONFIG_LOG_PLATFORM
+ *
+ * Define to enable platform region logging.
+ *
+ */
+#define OPENTHREAD_CONFIG_LOG_PLATFORM                          1
+
 /**
  * @def OPENTHREAD_CONFIG_NCP_TX_BUFFER_SIZE
  *