Add an ability to disable discover-ability of a plugin endpoint

[garagatyi]

Description

We use server discovery to be able to provide a seamless pluggability for Che 7 sidecars. On k8s/OS infras it is based on k8s Service. Which mean that Che plugin endpoint name has to be a valid k8s Service name. Since Che plugin endpoint names are allowed to include characters not allowed in k8s Service name we can't create a k8s Service for such Che plugin endpoint.
Since it is needed to start classic Che GWT IDE and it doesn't need service discovery at the moment we can add server attribute discoverable=false that would inform Che that we don't need to create k8s service with an equal name for a Che plugin endpoint and we won't have an incompatibility issue.
To implement that for Workspace.Next flow where service discovery is implemented we can add a check here whether we the endpoint has attribute discoverable and whether the value is false. And if so, do not create a k8s service for such an endpoint.

Reproduction Steps

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