feat(ltm): add file data support

ltm/include/ltm/common.h

@@ -24,6 +24,7 @@ typedef enum ltm_err {
   ltm_err_ok = 0,
   ltm_err_invalid_template,
   ltm_err_failed_alloc,
+  ltm_err_failed_io,
   ltm_err_not_found,
   ltm_err_wrong_block_type,
   ltm_err_done,

ltm/include/ltm/template.h

@@ -53,6 +53,8 @@ void ltm_instance_free(ltm_instance *instance);
 typedef enum ltm_instance_block_type {
   ltm_instance_block_type_buf = 0,
   ltm_instance_block_type_buf_owned,
+  ltm_instance_block_type_file,
+  ltm_instance_block_type_file_owned,
   ltm_instance_block_type_nested,
 } ltm_instance_block_type;