Add new argument to validateFields which enforces validation only on specified fields

[matteokov]

Hi,

I added a new argument to validateFields which specifies fields that need to be validated. Main usage for this should be when an entity has virtual fields that are not stored in DB. Right now even with validExistence set to false, you get an error because ClassMeta doesn't recognize virtual fields.

The default value is set to null or all fields, so we don't have any breaking changes.