chore: add Coder service prefix to tailnet (#14943)

re: #14715

This PR introduces the Coder service prefix: `fd60:627a:a42b::/48` and refactors our existing code as calling the Tailscale service prefix explicitly (rather than implicitly).

Removes the unused `Addresses` agent option. All clients today assume they can compute the Agent's IP address based on its UUID, so an agent started with a custom address would break things.

codersdk/workspacesdk/agentconn.go

@@ -51,7 +51,7 @@ type AgentConnOptions struct {
 }
 
 func (c *AgentConn) agentAddress() netip.Addr {
-	return tailnet.IPFromUUID(c.opts.AgentID)
+	return tailnet.TailscaleServicePrefix.AddrFromUUID(c.opts.AgentID)
 }
 
 // AwaitReachable waits for the agent to be reachable.

codersdk/workspacesdk/workspacesdk.go

@@ -236,7 +236,7 @@ func (c *Client) DialAgent(dialCtx context.Context, agentID uuid.UUID, options *
 			CompressionMode: websocket.CompressionDisabled,
 		})
 
-	ip := tailnet.IP()
+	ip := tailnet.TailscaleServicePrefix.RandomAddr()
 	var header http.Header
 	if headerTransport, ok := c.client.HTTPClient.Transport.(*codersdk.HeaderTransport); ok {
 		header = headerTransport.Header