Add a yellowstone stub and partially migrate denali

sys/yellowstone/include/yellowstone_stub.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <mammoth/endpoint_client.h>
+#include <mammoth/memory_region.h>
+#include <ztypes.h>
+
+class YellowstoneStub {
+ public:
+  explicit YellowstoneStub(z_cap_t yellowstone_cap);
+
+  glcr::ErrorOr<MappedMemoryRegion> GetAhciConfig();
+
+ private:
+  glcr::UniquePtr<EndpointClient> yellowstone_stub_;
+};