ipn, ipn/ipnlocal: add session identifier for WatchIPNBus

This PR adds a SessionID field to the ipn.Notify struct so that
ipn buses can identify a session and register deferred clean up
code in the future. The first use case this is for is to be able to
tie foreground serve configs to a specific watch session and ensure
its clean up when a connection is closed.

Updates #8489

Signed-off-by: Marwan Sulaiman <marwan@tailscale.com>
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