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.

coderd/tailnet_test.go

@@ -186,7 +186,9 @@ func TestServerTailnet_ReverseProxy(t *testing.T) {
 		// Ensure the reverse proxy director rewrites the url host to the agent's IP.
 		rp.Director(req)
 		assert.Equal(t,
-			fmt.Sprintf("[%s]:%d", tailnet.IPFromUUID(a.id).String(), workspacesdk.AgentHTTPAPIServerPort),
+			fmt.Sprintf("[%s]:%d",
+				tailnet.TailscaleServicePrefix.AddrFromUUID(a.id).String(),
+				workspacesdk.AgentHTTPAPIServerPort),
 			req.URL.Host,
 		)
 	})