Option to disable validation of unversioned paths

[ml-evs]

There is currently no option for disabling validation of the unversioned base URL, effectively making it mandatory. While validation of this URL can be useful (and is required to check for things like the versions endpoint) the optimade-validator itself still expects to find a full implementation at the unversioned URL, if that is what is passed to it.

Unless/until we change the validator to do the version negotiation for us (Materials-Consortia/optimade-python-tools#662) it would be helpful to have a setting that validates just the URL passed through GitHub actions.

[ml-evs]

Unless/until we change the validator to do the version negotiation for us (Materials-Consortia/optimade-python-tools#662) it would be helpful to have a setting that validates just the URL passed through GitHub actions.

Alternative framing: option to disable unversioned URL validation (perhaps even by default) so that only <base_url>/v1 gets validated by default.