helm: Make OAuth2 Proxy's allowed redirect domains configurable

[lucas-koehler]

Is your feature request related to a problem? Please describe.

The Theia Cloud Helm chart configures allowed redirect domains for session pods' OAuth proxies. This list is defined in the proxy's config map's whitelist_domains. This is currently hard coded to the landing page and session host. As this is also used for the redirects after logging out, only the landing page can be used as a redirect after logging out. This should be configurable.

Describe the solution you'd like

Extend the Helm chart to allow configuring the whitelist_domains and cookie_domains configurations. Also remove google.com from the default allowed redirects. This is likely a leftover.