Support matching any input

[eliobischof]

There is no support for matching ANY input. Each stub input is just valid if it has at least one equals/contains/matches rule. So you always have to specify a speficic key that is compared.

When I don't care about the input structure, I'd like to have a stub that matches on each call, regardless of the keys.
Also, a gRPC request does not necessarily have any keys. They can't be matched at all.