add CreateDirectChannel plugin api (#9848)

app/plugin_api_test.go

@@ -509,3 +509,21 @@ func TestPluginAPIRemoveTeamIcon(t *testing.T) {
 	err = api.RemoveTeamIcon(th.BasicTeam.Id)
 	require.Nil(t, err)
 }
+
+func TestPluginAPICreateDirectChannel(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+	api := th.SetupPluginAPI()
+
+	dm1, err := api.CreateDirectChannel(th.BasicUser.Id, th.BasicUser2.Id)
+	require.Nil(t, err)
+	require.NotEmpty(t, dm1)
+
+	dm2, err := api.CreateDirectChannel(th.BasicUser.Id, th.BasicUser.Id)
+	require.Nil(t, err)
+	require.NotEmpty(t, dm2)
+
+	dm3, err := api.CreateDirectChannel(th.BasicUser.Id, model.NewId())
+	require.NotNil(t, err)
+	require.Empty(t, dm3)
+}