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:

https://github.com/microsoft/component-detection/blob/368f3c9692a51795d4f89a46ff6061ded9109a11/src/Microsoft.ComponentDetection.Orchestrator/Services/DetectorProcessingService.cs#L69

Which eventually creates a new ActionBlock:

https://github.com/microsoft/component-detection/blob/368f3c9692a51795d4f89a46ff6061ded9109a11/src/Microsoft.ComponentDetection.Contracts/FileComponentDetector.cs#L92-L94

Recommended after speaking with @dfederm