res_stir_shaken: refactor utility function

Refactors temp file utility function into file.c.

ASTERISK-29809 #close

Change-Id: Ife478708c8f2b127239cb73c1755ef18c0bf431b

include/asterisk/file.h

@@ -146,6 +146,23 @@ int ast_filecopy(const char *oldname, const char *newname, const char *fmt);
  */
 FILE *ast_file_mkftemp(char *template, mode_t mode);
 
+/*!
+ * \brief Create a temporary file located at path
+ *
+ * \note The directory containing path will be created if it does not exist
+ * \note This function assumes path does not end with a '/'
+ *
+ * \param path The directory path to create the file in
+ * \param filename Function allocates memory and stores full filename (including path) here
+ * \param template_name mkstemp template to use. Must end with XXXXXX.
+ *
+ * \note filename will need to be freed with ast_free if this function succeeds
+ *
+ * \retval -1 on failure
+ * \return file descriptor on success
+ */
+int ast_file_fdtemp(const char *path, char **filename, const char *template_name);
+
 /*!
  * \brief Callback called for each file found when reading directories
  * \param dir_name the name of the directory