Multi addon projects

[kraenhansen]

This draft PR adds an ability for cmake-rn to build projects declaring multiple addons in a single invocation.

Currently drafted, since I need to figure out how to handle the final output.

The current state of this PR produce prebuilds located in packages/node-addon-examples/build instead of a ./build directory next to the individual source folders:

> ls RelWithDebInfo    
async-test-addon.android.node   example-1-hello.apple.node      example-4-addon.android.node    example-7-addon.apple.node
async-test-addon.apple.node     example-10-binding.android.node example-4-addon.apple.node      example-8-addon.android.node
buffers-test-addon.android.node example-10-binding.apple.node   example-5-addon.android.node    example-8-addon.apple.node
buffers-test-addon.apple.node   example-2-hello.android.node    example-5-addon.apple.node      example-9-addon.android.node
example-0-hello.android.node    example-2-hello.apple.node      example-6-addon.android.node    example-9-addon.apple.node
example-0-hello.apple.node      example-3-addon.android.node    example-6-addon.apple.node
example-1-hello.android.node    example-3-addon.apple.node      example-7-addon.android.node

[kraenhansen]

Superseded by #413, which carries these commits rebased onto next and resolves the output question this was drafted on: each addon is emitted next to the sources it was built from, named after the target's OUTPUT_NAME rather than the namespaced CMake target, so --namespaced-targets stays invisible to everything downstream.

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