Introduce base Camera class to support alternative camera implementations

This commit introduces a new 'Camera' base class positioned between the
API layer and the existing 'ESP32Camera' implementation.
- No changes to functionality in 'ESP32Camera' or
'ESP32CameraWebServer'.
- This refactoring enables future camera implementations to integrate
with the existing API.
- The goal is to keep the commit as minimal and non-breaking as
possible.

This is the first step in a series of changes aimed at modernizing and
generalizing ESPHome's camera support.

esphome/components/api/api_pb2_service.cpp

@@ -204,7 +204,7 @@ void APIServerConnectionBase::read_message(uint32_t msg_size, uint32_t msg_type,
       this->on_execute_service_request(msg);
       break;
     }
-#ifdef USE_ESP32_CAMERA
+#ifdef USE_CAMERA
     case 45: {
       CameraImageRequest msg;
       msg.decode(msg_data, msg_size);
@@ -682,7 +682,7 @@ void APIServerConnection::on_button_command_request(const ButtonCommandRequest &
   }
 }
 #endif
-#ifdef USE_ESP32_CAMERA
+#ifdef USE_CAMERA
 void APIServerConnection::on_camera_image_request(const CameraImageRequest &msg) {
   if (this->check_authenticated_()) {
     this->camera_image(msg);