Allow build settings declaration on Xcode target build tasks

[lacasseio]

As a user building Xcode target using Gradle, I want to declare additional build settings to pass to xcodebuild, so that I can manipulate the build settings seen by xcodebuild (as well as ensure optimal performance while Nokee adds support for Xcode build settings).

Acceptance Criteria

• Additional build settings can reference other build settings
• Additional build settings take precedence over xcodebuild default build settings according to the layering resolution

[lacasseio]

This story is mostly about working around the shortcoming of xcodebuild forking. We should convert our current "shortcut" resolution to this new layer, effectively making our shortcut build settings explicit build settings to xcodebuild. Users should be able to revert to a pure xcodebuild driven, by clearing the additional build settings. Note that the build settings to redirect the derived data may not be cleared given those are essential to the sharing of derived data directories.