fix: use client preferred URL for the default DERP (#18911)

The agentsdk currently does a remap of the DERP map to change the
EmbeddedRelay node's URL to match the agent's access URL.

This PR makes changes to the `workspacesdk` (used by clients like the
CLI) and `vpn` (used by Coder Desktop) to match this behavior.

This enables us the ability to try Coder clients in dogfood over a VPN
without changing the global access URL.

agent/agent.go

@@ -98,7 +98,7 @@ type Client interface {
 	ConnectRPC26(ctx context.Context) (
 		proto.DRPCAgentClient26, tailnetproto.DRPCTailnetClient26, error,
 	)
-	RewriteDERPMap(derpMap *tailcfg.DERPMap)
+	tailnet.DERPMapRewriter
 }
 
 type Agent interface {