api: sdvx tape led (#857)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #852

## Description of change
Add SDVX valk cab tape LED output over API

## Testing
tested with custom python script over api

src/spice2x/api/client.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <atomic>
+#include <cstdint>
+
+namespace api {
+
+    extern std::atomic_uint32_t CLIENT_COUNT;
+
+    inline bool has_clients() {
+        return CLIENT_COUNT.load(std::memory_order_relaxed) > 0;
+    }
+}