Provide a way for policies to override the IServiceConnector used for a request (needed for caching)

The caching policy needs a way to hook into the processing of the request such that it can prevent the engine from actually going out to the real back-end service.

To allow this, the policy needs a way to provide its own custom IServiceConnector, which gets the response (headers and content) from a cache. If a policy does this, then the engine will use that connector rather than create one to the back-end service.

Only one provided custom IServiceConnector should be allowed per request. If two policies try to provide this, then the second one should fail.