This patch comprises the API changes outlined in the Windows GMSA KEP
(https://github.com/kubernetes/enhancements/blob/master/keps/sig-windows/20181221-windows-group-managed-service-accounts-for-container-identity.md)
to add GMSA support to Windows workloads.

It includes validation, as well as dropping fields if the `WindowsGMSA` feature
flag is not set, both with unit tests.
Signed-off-by: Jean Rouge <rougej+github@gmail.com>