Use single shared `ActionBlock`

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

Start in src/Microsoft.ComponentDetection.Orchestrator/Services/DetectorProcessingService.cs around line 69, then trace the ActionBlock creation in src/Microsoft.ComponentDetection.Contracts/FileComponentDetector.cs around lines 92-94. Determine how detector processing currently creates per-detector blocks and refactor it to use one shared ActionBlock across all detectors. Done means the shared processing behavior is preserved without creating a separate block for each detector.

Written by the indexing model from the issue text.