feat: Server: Add support for previews via sdcpp API (#2093)

Co-authored-by: leejet <leejet714@gmail.com>

src/core/util.h

@@ -101,6 +101,7 @@ std::vector<std::pair<std::string, float>> split_quotation_attention(
 sd_progress_cb_t sd_get_progress_callback();
 void* sd_get_progress_callback_data();
 
+void sd_begin_preview_pass(int total_steps);
 sd_preview_cb_t sd_get_preview_callback();
 void* sd_get_preview_callback_data();
 preview_t sd_get_preview_mode();