Introduce base Camera class to support alternative camera implementations (#9285)

Co-authored-by: J. Nick Koston <nick@koston.org>
Co-authored-by: J. Nick Koston <nick+github@koston.org>

esphome/components/camera/camera.cpp

@@ -0,0 +1,22 @@
+#include "camera.h"
+
+namespace esphome {
+namespace camera {
+
+// NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
+Camera *Camera::global_camera = nullptr;
+
+Camera::Camera() {
+  if (global_camera != nullptr) {
+    this->status_set_error("Multiple cameras are configured, but only one is supported.");
+    this->mark_failed();
+    return;
+  }
+
+  global_camera = this;
+}
+
+Camera *Camera::instance() { return global_camera; }
+
+}  // namespace camera
+}  // namespace esphome