implement "has" operator to check for key existence

[rhrazdil]

In a scenario where user would like to configure for example all ethernet NICs that are not veth, nmpolicy should
implement has or exists operator.

Currently, the python implementation of nmstatectl outputs type: veth for veth interfaces, but future rust implementation
outputs

type: ethernet
veth:
  peer: name

So for filtering ethernet interfaces that are not veths, we'll need to check for existence of veth key.

[qinqon]

Instead of new operator we can create a new "none" literal used to check field existence

interfaces.veth == none

If we go with "exists" we will need to also implement != operator

interfaces.veth != exists

[rhrazdil]

Ah, I assumed that != is alredy implemented. I think using none literal is a good suggestion. I'll go with that