tailcfg, control/controlclient: add tailcfg.PeersChangedPatch [capver 33]

This adds a lighter mechanism for endpoint updates from control.

Change-Id: If169c26becb76d683e9877dc48cfb35f90cc5f24
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f