vz: warn ignored YAML fields

[AkihiroSuda]

Warnings should be printed for ignored YAML propertries: https://github.com/containerd/nerdctl/blob/c1a4159035b77a4a739a503b98139a17ac3440b8/pkg/composer/serviceparser/serviceparser.go#L38

Can be another PR though

Originally posted by @AkihiroSuda in #1147 (comment)

[balajiv113]

I will work on this next