Allow baseline to be omitted for builtin registries

[sweemer]

Is your feature request related to a problem? Please describe.
Currently the baseline field is always required for builtin registries as described here. In my case, the baseline should always be the same version as the built-in registry, which is a git submodule of my project. Having to explicitly specify it in vcpkg-configuration.json opens up the possibility for inconsistencies if somebody forgets to update it after updating the submodule.

Proposed solution
Make baseline an optional field for builtin registries. If omitted, the baseline will be the same version as the built-in registry itself.

Describe alternatives you've considered
None that come to mind.

Additional context
None