Allow "maximum versioning" for custom registries.

[zcotman]

We have recently started to use vcpkg where I work. We are using a custom registry for our internal packages. We would prefer to take a "live at head" approach for these packages. Now, because vcpkg uses a "minimum versioning" scheme, we have to make commits to repos for dependents updating the baselines when we make an update to a package in the registry. It would save quite a few commits if we could set a flag such as "versioning scheme" to "maximum versioning" for our custom registry (in the configuration file) which would trigger vcpkg to collect the highest version available in the registry instead of the minimum version associated with the baseline. Minimum versioning makes a lot of sense as a default, but, especially for custom registries, allowing a "live at head" approach could be very nice.

If anyone has suggestions for a different way to "live at head" with vcpkg and our internal tools, I would be happy to hear them.