Fix #498

Introduces Scatter/Gather support. Given an incoming HTTP request matching the configured template, the request will be fanout to downstream endpoints identified by each Gatherer.

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

PoA

• PR description
• Apply labels as appropriate
• tests
  • Basic GET
  • Query string propagation
  • Works with other composition
  • Custom aggregators
  • Custom gatherers
  • What happens if no HttpClient is registered in DI
• Documentation
  • aggregators
  • Link from docs README
  • mention in repo README