MM-27493 Shared channels (MVP) (#17301)

Remote Cluster Service
- provides ability for multiple Mattermost cluster instances to create a trusted connection with each other and exchange messages
- trusted connections are managed via slash commands (for now)
- facilitates features requiring inter-cluster communication, such as Shared Channels
Shared Channels Service
- provides ability to shared channels between one or more Mattermost cluster instances (using trusted connection)
- sharing/unsharing of channels is managed via slash commands (for now)

app/session_test.go

@@ -439,3 +439,39 @@ func TestGetCloudSession(t *testing.T) {
 		require.Equal(t, "api.context.invalid_token.error", err.Id)
 	})
 }
+
+func TestGetRemoteClusterSession(t *testing.T) {
+	th := Setup(t)
+	token := model.NewId()
+	remoteId := model.NewId()
+
+	rc := model.RemoteCluster{
+		RemoteId:     remoteId,
+		RemoteTeamId: model.NewId(),
+		DisplayName:  "test",
+		Token:        token,
+		CreatorId:    model.NewId(),
+	}
+
+	_, err := th.GetSqlStore().RemoteCluster().Save(&rc)
+	require.NoError(t, err)
+
+	t.Run("Valid remote token should return session", func(t *testing.T) {
+		session, err := th.App.GetRemoteClusterSession(token, remoteId)
+		require.Nil(t, err)
+		require.NotNil(t, session)
+		require.Equal(t, token, session.Token)
+	})
+
+	t.Run("Invalid remote token should return error", func(t *testing.T) {
+		session, err := th.App.GetRemoteClusterSession(model.NewId(), remoteId)
+		require.Error(t, err)
+		require.Nil(t, session)
+	})
+
+	t.Run("Invalid remote id should return error", func(t *testing.T) {
+		session, err := th.App.GetRemoteClusterSession(token, model.NewId())
+		require.Error(t, err)
+		require.Nil(t, session)
+	})
+}