Make list of support generic diagnostc listeners configurable

[lmolkova]

DependencyCollector now has a hardcoded list of supported dependencies (http only for now):

namespace OpenCensus.Collector.Dependencies{
       // ...
       public DependenciesCollector(DependenciesCollectorOptions options, ITracer tracer, ISampler sampler, IPropagationComponent propagationComponent)
        {
            this.diagnosticSourceSubscriber = new DiagnosticSourceSubscriber(
                new Dictionary<string, Func<ITracer, Func<HttpRequestMessage, ISampler>, ListenerHandler>>()
                {
                    { "HttpHandlerDiagnosticListener", (t, s) => new HttpHandlerDiagnosticListener(t, s, propagationComponent) } ,
                   // e.g. how do I add { "Microsoft.Azure.Sdk", (t, s) => new AzureSdkListener(t, sampler) }
                },
                tracer,
                x =>  {... });
            this.diagnosticSourceSubscriber.Subscribe();
        }

We should allow configuring generic sources by name and allowing to provide custom handlers. This could be done via DependenciesCollectorOptions.

This is also useful when users need to remove default collectors.

[SergeyKanzhelev]

Moved to OpenTelemetry