Refactor MSUnmerged frontend rules (rse_type)

Impact of the new feature CMSWEB redirection rules

Is your feature request related to a problem? Please describe. When MSUnmerged was commissioned in CMSWEB, we needed to have a mechanism to reach specific ms-unmerged replicas, since their status and RSE data was only available in the instance memory itself. Some relevant deployment changes can be seen in: https://github.com/dmwm/deployment/pull/1075 https://github.com/dmwm/deployment/pull/1078

Now that the service has been made stateless, using MongoDB as a backend, we can remove those query string tweaks from the backends map, and simple let nginx decide which ms-unmerged replica will serve a client request.

Describe the solution you'd like Now that there is a MongoDB database behind MSUnmerged, we can change the logic between frontends and backends, like:

completely remove the rse_type query string from the url matchmaking (in the backends txt file)
discuss with Imran if and how we can have a random/round-robin mechanism to have any of the MSUnmerged k8s replicas to be chosen to serve a request made by a client.

Describe alternatives you've considered Not change anything and keep the current behavior, which is:

in testbed: the query string decides which replica serves a given request
in production: there is a hard-wired k8s replica to serve any client requests.

Additional context None

dmwm / WMCore

Refactor MSUnmerged frontend rules (rse_type) #10942