Scatter/Gather support

[mauroservienti]

A potential user's use case is to compose a set of responses from different sources. Those responses are structurally all the same.

It would be possible to expose a configuration API like the following that, under the hood, would automatically fan out to the configured set of desintation, and composes back the list of responses before returning to the client:

app.UseEndpoints(builder =>
{
    builder.MapScatterGather(template: "/source", new ScatterGatherOptions
    {
        Gatherers = new List<Gatherer>
        {
            new Gatherer()
            {
                Key = "sample",
                Destination = "http://foo.bar/source"
            }
        }
    });
});