NewMatcher creates a Matcher composed of all of the registered matchers.

Cookie is added to a `Set-Cookie` header in the mock response.

Matcher is the core service that orchestrates comparing the incoming request against the matchers and returning the response if the request is matched.

MatchRequest is the user defined matcher that we check incoming requests against.

Mock represents an individual mock.

Response is returned to the consumer if the MockRequest matches.

TODO: the path arg here is a leaky abstraction - fix it.