Remove Observables and switch to Channels

[melotic]

Our current implementation for scanning files/directories is done by using observables:

component-detection/src/Microsoft.ComponentDetection.Common/FastDirectoryWalkerFactory.cs

Line 32 in 29e8952

return Observable.Create<FileSystemInfo>(s =>

Instead, we should architect a channel system that works as:

1. Create a new Task & Channel for each detector
2. Launch each detector with a semphore blocking execution untill all detectors have subscribed to the channel
3. Begin scanning directories
4. Each file found will get sent through the channel
5. Detector gets the message, and scans file or ignores it depending on the pattern

We could also implement a Broadcast channel similar to https://docs.rs/tokio/latest/tokio/sync/index.html#broadcast-channel

AB#2102424