tailcfg,wgengine: add initial support for WireGuard only peers

A peer can have IsWireGuardOnly, which means it will not support DERP or
Disco, and it must have Endpoints filled in order to be usable.

In the present implementation only the first Endpoint will be used as
the bestAddr.

Updates tailscale/corp#10351

Co-authored-by: Charlotte Brandhorst-Satzkorn <charlotte@tailscale.com>
Co-authored-by: James Tucker <james@tailscale.com>
Signed-off-by: James Tucker <james@tailscale.com>
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