Circuit breaking middleware

[Nevon]

Mappersmith ships with some commonly needed middleware, such as for retrying. One capability that it does not ship with is circuit breaking. This is a generic and I believe common need that users of Mappersmith will likely end up implementing themselves.

We have an internal middleware for doing circuit breaking that relies on opossum for the actual circuit breaking logic. I don't think adding an additional dependency into mappersmith for a middleware that may or may not be used makes sense, but I also don't think the circuit breaking logic itself needs to be as complex as what opossum implements. I haven't yet explored what an implementation of such a middleware would look like, so I might be wrong, but I wanted to open this feature request to gauge the appetite for adopting a built-in middleware.