Fix Dialyzer by copying some type specs that Jiffy doesn't export

Since Erlang/OTP 26.0, Dialyzer enables by default "unknown" types warnings
https://www.erlang.org/patches/otp-26.0

include/mod_matrix_gw.hrl

@@ -26,3 +26,19 @@
          implicit_room_creator :: boolean(),
          updated_redaction_rules :: boolean()
         }).
+
+%% Jiffy doesn't export those type specs, so they are copied here from
+%% https://github.com/davisp/jiffy/blob/master/src/jiffy.erl
+
+-type json_value() :: null
+                    | true
+                    | false
+                    | json_string()
+                    | json_number()
+                    | json_object()
+                    | json_array().
+-type json_array()  :: [json_value()].
+-type json_string() :: atom() | binary().
+-type json_number() :: integer() | float().
+-type json_object() :: {[{json_string(),json_value()}]}
+                        | #{json_string() => json_value()}.

src/mod_matrix_gw.erl

@@ -736,7 +736,7 @@ sign_json(Host, JSON) ->
         binary(),
         [binary()],
         [{binary(), binary()}],
-        none | jiffy:json_object(),
+        none | json_object(),
         [any()],
         [any()]) -> {ok, any()} | {error, any()}.
 

src/mod_matrix_gw_room.erl

@@ -67,7 +67,7 @@
          sender :: binary(),
          prev_events :: [binary()],
          origin_server_ts :: integer(),
-         json :: jiffy:json_object(),
+         json :: json_object(),
          state_map}).
 
 -record(data,