Add CA cert secret to the installer

[csweichel]

To support "self-signed" certs within Gitpod we need to provide means for users to supply their CA cert.

To this end, we'd introduce a new field in the installer config where users can reference a secret which carries their CA cert chain. We'd add cluster validation which ensures that if this secret is supplied it carries a ca.crt entry.

This secret would be consumed by #9078 and #9079