[proxy,client] Add --external-ip-map flag for the embedded client (#8185)

* [proxy] Add --external-ip-map flag for the embedded client

The proxy behind a NAT with a forwarded fixed --wg-port could not advertise
its public address as an ICE candidate, so peers fell back to relay. Expose
the agent's external IP map through embed.Options and a proxy flag
(NB_PROXY_EXTERNAL_IP_MAP).

* [proxy] Validate --external-ip-map at startup

The engine drops every external IP mapping when one fails to parse, so a
typo or a space after a comma silently disabled the feature. Move the
agent's validator to shared/externalipmap, run it from the proxy at
startup, and trim entries first.

* [client,proxy] Require an external IP first in --external-ip-map

The engine reads the first component as the external IP, so a mapping like
iface/10.0.0.5 passed validation and then dropped every mapping.

client/embed/embed.go

@@ -118,6 +118,9 @@ type Options struct {
 	MTU *uint16
 	// DNSLabels defines additional DNS labels configured in the peer.
 	DNSLabels []string
+	// NATExternalIPs maps external IPs to internal IPs or interfaces for ICE
+	// candidates, same format as the agent's --external-ip-map flag.
+	NATExternalIPs []string
 	// Performance configures the tunnel's buffer pool cap and batch size.
 	Performance Performance
 }
@@ -224,6 +227,7 @@ func New(opts Options) (*Client, error) {
 		WireguardPort:       opts.WireguardPort,
 		MTU:                 opts.MTU,
 		DNSLabels:           parsedLabels,
+		NATExternalIPs:      opts.NATExternalIPs,
 	}
 	if opts.ConfigPath != "" {
 		config, err = profilemanager.UpdateOrCreateConfig(input)