Make onion-key body optional in microdescs

Also, stop storing onion keys in microdesc_t.

(In prop350, for microdescs, we are making the body optional; the "onion-key"
entry is still mandatory, so that we can tell where microdescs begin.)

src/feature/dirparse/parsecommon.h

@@ -220,6 +220,7 @@ typedef struct directory_token_t {
 typedef enum {
   NO_OBJ,        /**< No object, ever. */
   NEED_OBJ,      /**< Object is required. */
+  OPT_KEY_1024,  /**< If object is present, it must be a 1024 bit public key */
   NEED_KEY_1024, /**< Object is required, and must be a 1024 bit public key */
   NEED_KEY,      /**< Object is required, and must be a public key. */
   OBJ_OK,        /**< Object is optional. */