Allow use of `sessionStorage` in `browserSessionIntegration` to more accurately count sessions for MPAs

### Description

Session updates we send from the Browser SDK are currently per pageview, leading to more sessions than is commonly understood for MPAs. We find that Sessions likely should have the same lifetime as a Session Replay session. For this we should introduce an option in the `browserSessionIntegration` that allows a user to configure a persistence model using `sessionStorage` to more accurately collect number of sessions.