[FL-2797] Signal Generator app (#1793)

* Signal Generator app
* MCO pin initialization in app
* furi_hal_pwm documentation

Co-authored-by: あく <alleteam@gmail.com>

applications/plugins/signal_generator/views/signal_gen_pwm.h

@@ -0,0 +1,21 @@
+#pragma once
+
+#include <gui/view.h>
+#include "../signal_gen_app_i.h"
+
+typedef struct SignalGenPwm SignalGenPwm;
+typedef void (
+    *SignalGenPwmViewCallback)(uint8_t channel_id, uint32_t freq, uint8_t duty, void* context);
+
+SignalGenPwm* signal_gen_pwm_alloc();
+
+void signal_gen_pwm_free(SignalGenPwm* pwm);
+
+View* signal_gen_pwm_get_view(SignalGenPwm* pwm);
+
+void signal_gen_pwm_set_callback(
+    SignalGenPwm* pwm,
+    SignalGenPwmViewCallback callback,
+    void* context);
+
+void signal_gen_pwm_set_params(SignalGenPwm* pwm, uint8_t channel_id, uint32_t freq, uint8_t duty);