Add `webservice` as an option for authentication enum

[pythrick]

Just like stated in ShinyProxy doc:

Amongst the different authentication methods, ShinyProxy also offers the possibility to use a custom web service that handles authentication with a HTTP POST call returning a session id and user information.

The following enumerators should be updated: https://github.com/openanalytics/shinyproxy-operator/blob/master/docs/deployment/bases/clustered/operator/crd.yaml#L60 https://github.com/openanalytics/shinyproxy-operator/blob/master/docs/deployment/bases/clustered/operator/crd.yaml#L199 https://github.com/openanalytics/shinyproxy-operator/blob/master/docs/deployment/bases/namespaced/operator/crd.yaml#L60 https://github.com/openanalytics/shinyproxy-operator/blob/master/docs/deployment/bases/namespaced/operator/crd.yaml#L199