[FR]: Synchronous getter for Session ID and a callback for when it changes

### Description

If possible please make a synchronous getter for SessionID and ideally also a callback/trigger for when it changes. 
I honestly find it very odd that we would need to keep track of the session timeout, or just end up calling an async function...

### API Proposal

_No response_

### Firebase Product(s)

Analytics

### Targeted Platform(s)

Apple Platforms, Android, Desktop