Added support for bypassing the content pipeline in MGCB

At first glance this PR sounds nonsensical--why would anyone want to bypass content building in a content builder? In short, because we're basically emulating the functionality of the Visual Studio-powered content pipeline, where a common pattern is to just copy an asset to the target directory.

With Visual Studio, you'd include the file in your content project and set the Build Action to either None or Content. In this case you'd add a command to the MGCB response file:
```
/copy:myAssetDir/myAssetFile.raw
```
The file will be copied to a mirrored directory structure in the output directory.