Add middleware example

[FranzBusch]

This PR adds an example showcasing how middleware can be used to intercept requests handled by a server and performed by a client. In detail, this PR adds a logging and request handling middleware. The logging middleware is capable of inspecting every single part of the request including the individual chunks read.

[czechboy0]

This is awesome - the logging middleware is a great example of a fully powerful middleware that also inspects individual body chunks. Though it does make it appear pretty complicated - I wonder if you could add (in a subsequent PR) a simpler middleware example that doesn't need to implement its own reader/writer, and only inspects the headers/trailers, so in theory should be much simpler.

[gjcairo]

Would it make sense to allow for separate Return types in next and in intercept? What if a middleware wants to transform next's return value into something else?

[FranzBusch]

I am not sure that transforming the Return type would work since in the end this is the type that a user returns from their perform closure. Let's keep it to one type for now.

[gjcairo]

Should these be sending too?

[FranzBusch]

It would require us to take the sender as sending right now I didn't need that. Let's keep an eye if for some middleware we want this.

[gjcairo]

Should these be sending?

[FranzBusch]

Same here