Use single shared `ActionBlock`

[JamieMagee]

Currently we're creating a single ActionBlock per detector, however we should look into using one shared ActionBlock across all detectors.

We're running each detector here:

component-detection/src/Microsoft.ComponentDetection.Orchestrator/Services/DetectorProcessingService.cs

Line 69 in 368f3c9

() => detector.ExecuteDetectorAsync(new ScanRequest(detectionArguments.SourceDirectory, exclusionPredicate, this.Logger, detectorArguments, detectionArguments.DockerImagesToScan, componentRecorder)),

Which eventually creates a new ActionBlock:

component-detection/src/Microsoft.ComponentDetection.Contracts/FileComponentDetector.cs

Lines 92 to 94 in 368f3c9

	private async Task<IndividualDetectorScanResult> ProcessAsync(IObservable<ProcessRequest> processRequests, IDictionary<string, string> detectorArgs)
	{
	var processor = new ActionBlock<ProcessRequest>(async processRequest => await this.OnFileFound(processRequest, detectorArgs));

Recommended after speaking with @dfederm