feat: advertise the low-bandwidth proxy in the /versions response

[Sorunome]

Signed-Off-By: Sorunome sorunome@famedly.com

Example response:

sorunome@sorunome-laptop backend/lb $ curl https://pixelthefox.net/_matrix/client/versions | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   183  100   183    0     0   2679      0 --:--:-- --:--:-- --:--:--  2691
{
  "versions": [
    "r0.5.0",
    "r0.6.0"
  ],
  "unstable_features": {
    "org.matrix.e2e_cross_signing": true
  },
  "org.matrix.msc3079.low_bandwidth": {
    "dtls": 4444,
    "cbor_enum_version": 1,
    "coap_enum_version": 1
  }
}