feat: add onNewSession function

[kdastan]

I've added simple function onNewSessionStart using which we can send callback function. On new session start provided callback will be called which can help detect new session start moment on our app.

Closes #429

[kevinpagtakhan]

Hey @kdastan, we included some changes you have here in a bigger change we are making therefore, we are closing this in favor of #455.