khepri_machine: Define the Ra machine version to 0

[Why]
This is not required at this point. However, users can query the machine
version without having to worry if the callback is defined (version = 0
when the callback is missing) or not.

[How]
We define the first version to be 0.

As mentionned in the patch, this version, even though it is valid, can't
be returned because the callback spec is too restrictive. Once the spec
is fixed, we can remove the Dialyzer directive added by this commit.

src/khepri_machine.erl

@@ -50,7 +50,8 @@
          handle_aux/6,
          apply/3,
          state_enter/2,
-         overview/1]).
+         overview/1,
+         version/0]).
 
 %% For internal use only.
 -export([clear_cache/1,
@@ -74,6 +75,13 @@
 
 -compile({no_auto_import, [apply/3]}).
 
+%% Turn off Dialyzer for `version/0'. It returns a version of 0 but it not
+%% allowed by the callback spec as of this writing. See
+%% https://github.com/rabbitmq/ra/pull/399 for the pull request which fixes
+%% this. Once there is a release of Ra with this patch, the following directive
+%% can be removed.
+-dialyzer({nowarn_function, version/0}).
+
 -type props() :: #{payload_version := khepri:payload_version(),
                    child_list_version := khepri:child_list_version()}.
 %% Properties attached to each node in the tree structure.
@@ -1316,6 +1324,9 @@ overview(#?MODULE{config = #config{store_id = StoreId},
       triggers => Triggers,
       keep_while_conds => KeepWhileConds}.
 
+version() ->
+    0.
+
 %% -------------------------------------------------------------------
 %% Internal functions.
 %% -------------------------------------------------------------------