Allow resource handling to be overridden

[pelson]

Currently it is not possible to override the resource handling, and the current implementation of streaming is not always desirable (in fact, it is probably only desirable when you have a local cache, or you have a network segmented setup).

At the very least, it would be good to be able to override the HttpResponse handling, such that we could choose to return a RedirectResponse, and avoid having the streaming overhead on every simple-repository-server.

Perhaps something like a base class:

class ResourceHandler:
    def handle_HttpResponse(...)

Such that you can easily override it would be the simplest way to go

[pelson]

The alternative to doing this: Always redirect if you get a URL resource. If you don't want redirection, we make it possible to add a repository component which streams URLs directly.
To do this, we need to have the ability to have a stream of bytes, but we already know we need this capability if we want to do resource creation on demand (or more precisely, the ability to stream bytes that aren't in a file or URL).

I think taking this alternative approach will simplify simple-repository-server, and in general will improve the API for get_resource, which today is a bit uncomfortable as it always results in a case statement.