Add support for Custom VirtualWorkspaces in listener and gateway

[nexus49]

Description

The goal is to mount a config map as a file to the gateway/listener that contains a list of virtual workspaces eg.:

virtualWorkspaces:
- name: example
  url: https://whatever./services/core.abc.com
- ...

The listener should process the virtualWorkspaces in the very same way as other workspaces and the user should be able to access the virtual workspace. A proposal is to access them in the following url pattern:

Normal Workspace: /kubernetes-graphql-gateway/root:abc:abc/query
Virtual Workspace: /kubernetes-graphql-gateway/virtualworkspace/example/query

This is a subtask to: #64