ipn,wgengine/magicsock: allow setting static node endpoints via tailscaled configfile (#12882)

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>

ipn/conf.go

@@ -42,6 +42,10 @@ type ConfigVAlpha struct {
 	AutoUpdate      *AutoUpdatePrefs `json:",omitempty"`
 	ServeConfigTemp *ServeConfig     `json:",omitempty"` // TODO(bradfitz,maisem): make separate stable type for this
 
+	// StaticEndpoints are additional, user-defined endpoints that this node
+	// should advertise amongst its wireguard endpoints.
+	StaticEndpoints []netip.AddrPort `json:",omitempty"`
+
 	// TODO(bradfitz,maisem): future something like:
 	// Profile map[string]*Config // keyed by alice@gmail.com, corp.com (TailnetSID)
 }