Fixes#8224
This PR adds a new `OpenVSX` object into the top-level `Config` field
to support configurations of the `openvsx-proxy` component. Currently,
Only `URL` field is present. This is needed to support air-gap
instlalations where people are expected to host their own open-vsx.
This config is top-level and not under the workspace or IDE as this
configures the proxy, but not the IDE.
The URL is validated by using the validate go tags.
Signed-off-by: Tarun Pothulapati <tarun@gitpod.io>
