[validation-gen] Add key and elem validation support to subfield validator

[aaron-prindle]

What would you like to be added?

Current the subfield tag/validator + //subfield(subfield-json-name)=<validator-arg> only supports field validators. It should be extended to support key and elem validators as well

Why is this needed?

This is needed as there are known validations that will go on fields that are slices, maps, arrays, etc. of a PodSpec which is embedded in a # of objects and as such needs to use subfield. This requires extending subfield to support key and element validators