routesrv route listing changes Etag more often than needed

[szuecs]

Describe the bug

If you use routesrv and the client you want to have at best a stable Etag to limit work on the control plane part of the data plane.
Routes and Predicates are sorted to mitigate this, but if multiple routes have the same predicate with different value, then it seems not sort stable.

Example:

% curl -I http://127.0.0.1:9990/routes
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1041476
Content-Type: text/plain; charset=utf-8
Etag: "0a860e4c0868b48977f322eaf6eff0bee1d8e0d5191c9b1f1654bb963813777d"
Last-Modified: Wed, 16 Aug 2023 10:42:22 GMT
X-Count: 1225
Date: Wed, 16 Aug 2023 10:42:23 GMT

% curl -I http://127.0.0.1:9990/routes
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1041476
Content-Type: text/plain; charset=utf-8
Etag: "0aa578823d84defb968ceba18eb7b14646441f9d8ceab0726ec8a150be0c2b28"
Last-Modified: Wed, 16 Aug 2023 10:42:25 GMT
X-Count: 1225
Date: Wed, 16 Aug 2023 10:42:26 GMT

% curl -I http://127.0.0.1:9990/routes
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1041476
Content-Type: text/plain; charset=utf-8
Etag: "0aa578823d84defb968ceba18eb7b14646441f9d8ceab0726ec8a150be0c2b28"
Last-Modified: Wed, 16 Aug 2023 10:42:25 GMT
X-Count: 1225
Date: Wed, 16 Aug 2023 10:42:27 GMT

% curl -I http://127.0.0.1:9990/routes
HTTP/1.1 200 OK
Accept-Ranges: bytes
Content-Length: 1041476
Content-Type: text/plain; charset=utf-8
Etag: "fabcdda1761dcfa4417e8f9841f308ce1f0d3778971b70fa4fb1b55c73d80638"
Last-Modified: Wed, 16 Aug 2023 10:42:34 GMT
X-Count: 1225
Date: Wed, 16 Aug 2023 10:42:35 GMT

A minimal example that I guess would flap is this:

r1: Header("A") -> <shunt>;
r2: Header("B") -> <shunt>;

or can be listed like this:

r1: Header("B") -> <shunt>;
r2: Header("A") -> <shunt>;

To Reproduce

internal only

Expected behavior

stable sorting is expected

Observed behavior

unstable sorting

[MustafaSaber]

Related PRs if someone wants to take over #2208 & #2202