`Divisor` `resource.Quantity` `omitempty` doesn't make sense

[regnaio]

https://github.com/kubernetes/api/blob/master/core/v1/types.go#L2309

When you json.Marshal() a ResourceFieldSelector with undefined Divisor, you get divisor: "0"

This is bad for 2 reasons:

1. omitempty is meaningless here
2. According to the docs, Divisor should default to 1. Since it actually defaults to 0, you get unintended behavior and incorrect results

Divisor instead should be a pointer *resource.Quantity instead of just resource.Quantity

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/lifecycle stale

[ciphx]

/assign