Define posix radio RadioSpinel accessor (#9018)

When adding cli commands to be used by a posix platform is is useful
to have access to the spinel interface when trying to communicate with
a RCP using vendor specific spinel properties. It is inadvisable to
create a separate spinel interface for custom properties as the two
interfaces would need to avoid contention on the hardware interface.

This accessor method allows access to the static spinel interface
defined in radio.cpp for external code segments. Dependent source
files can do so with: `(RadioSpinel<InterfaceType,
ProcessContextType>*)Posix::Radio::getSpinelInstance()` or similar.

src/posix/platform/radio.cpp

@@ -201,6 +201,8 @@ void Radio::Init(void)
 #endif // OPENTHREAD_CONFIG_PLATFORM_RADIO_COEX_ENABLE
 }
 
+void *Radio::GetSpinelInstance(void) { return &sRadioSpinel; }
+
 } // namespace Posix
 } // namespace ot
 

src/posix/platform/radio.hpp

@@ -55,6 +55,14 @@ public:
      */
     void Init(void);
 
+    /**
+     * This method acts as an accessor to the spinel instance used by the radio.
+     *
+     * @returns A pointer to the radio's spinel interface instance.
+     *
+     */
+    static void *GetSpinelInstance(void);
+
 private:
     RadioUrl mRadioUrl;
 };