wgengine/magicsock,ipn: allow setting static node endpoints via tailscaled config file.
Adds a new StaticEndpoints field to tailscaled config
that can be used to statically configure the endpoints
that the node advertizes. This field will replace
TS_DEBUG_PRETENDPOINTS env var that can be used to achieve the same.
Additionally adds some functionality that ensures that endpoints
are updated when configfile is reloaded.
Also, refactor configuring/reconfiguring components to use the
same functionality when configfile is parsed the first time or
subsequent times (after reload). Previously a configfile reload
did not result in resetting of prefs. Now it does- but does not yet
tell the relevant components to consume the new prefs. This is to
be done in a follow-up.
Updates tailscale/tailscale#12578
Signed-off-by: Irbe Krumina <irbe@tailscale.com>