Utilize CustomExecution build task once API is stabilized

[jorticus]

Currently build tasks are not very flexible - tasks.json must exactly match all provided functions, so you cannot customize build arguments for example.

We should consider moving to the CustomExecution API once it makes it out of insider:
https://code.visualstudio.com/updates/v1_33

This in theory should allow us to take in additional options before spawning the build task, and allow the user to configure arguments passed to the build script.

[jorticus]

This would mean we could get rid of the ${config:musupport.currentPlatformBuildScriptPath} hack, which allows us to determine which build script to use when invoking the build task.

[matthewfcarlson]

I like it. It would allow the codebase to specify whether to use mu tools or edk2 tools