[Apple] Add prebuilt dynamic framework support

Created by: robbertvanginkel

This PR proposes a way to embed prebuilt dynamic frameworks into apple_bundles, linking that framework against the binary for the apple bundle. This is a start to solving #440 (closed), in buck it (mostly) works, the project generation still has to be done. The rule syntax for this would be:

prebuilt_apple_framework(
    name = 'BuckTest',
    framework = 'BuckTest.framework',
)

You can add this rule to an apple_library or apple_binary, this should do the following:

• Symlink the Headers folder inside the framework as the exposed headers for the rule.
• Export linkerflag -rpath @loader_path/Frameworks/ -rpath @executable_path/Frameworks
• Export the linkerflag -framework <frameworkname>, and -F path/to/framework/
• Embed the framework in the bundle's "Frameworks" folder

I'm however encountering the following problem when trying to embed the framework: the INCLUDE_FRAMEWORKS flavor is tied to apple_library's FRAMEWORK flavor, and I don't want that. This happens in AppleDescriptions.java. I have the following situation: an apple_binary (wrapped in apple_bundle) with two dependencies: a prebuilt_apple_framework and an apple_libary. I want to statically link the apple_library, and embed and dynamically link the prebuilt_apple_framework. The probelm is, when I build the apple_binary with #INCLUDE_FRAMEWORKS, the mentioned code in AppleDescriptions goes through the deps and requires them with the FRAMEWORK_FLAVOR, which tries to embed the apple_library as a framework which is broken.

I sort of avoided this by making apple_binary not require the framework flavor, but this is inconsistent and will cause headaches. Suggestions on how to move forward with this?