[installer] Make OpenVSX URL configurable

[corneliusludmann]

If users want to use their own OpenVSX instance (e.g. in an airgap installation), they need to configure their own OpenVSX URL.

[mrsimonemms]

As part of this task, we should generate a new version of the example-config.yaml for the repo

gitpod-installer init > example-config.yaml