Add basic validation for PLZ resources

[yorugac]

Currently, PLZ takes into account only resources.limits field: that field is sent to GCk6 upon PLZ registration. But it is possible to declare a PLZ resource with resources.requests instead. Such registration then fails with an opaque 400 Validation error which is pretty hard to troubleshoot without knowing this trick.

As the simplest solution, we could add some basic explicit validation in k6-operator to check for empty resources.limits and log the error early, without making a registration attempt. As a more complex but better solution, it also makes sense to check if it's possible to add Kubernetes validation without breaking the current schema.