feat: add default special limit

[NikolaMilosa]

At the moment we can't easily have a default special limit (like having one Dfinity owned node per subnet) and then have an easy way to override it for a specific subnet (like having three Dfinity owned nodes in NNS). With this feature we can do that.

What we also get is the ability to override specific values for other potential new subnets. We could for e.g. turn off the requirement for a Dfinity owned node by saying adding an entry:

{ ...
        "default": {"node_provider": {"DFINITY": (1, "eq")}},
        "new-subnet": {"node_provider": {"DFINITY": (0, "eq")}},
}