feat(darwin): add L2CAP support

[acouvreur]

Add L2CAP CoC support for Darwin.

You can now open an L2CAP CoC to a device to send data and read directly at the L2CAP layer.

This is a work in progress and needs some design review.

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My proposition is that there is also a common interface for L2CAP Channels that implement io.ReadWriteCloser.

As a client only, you could:

• Read(p []byte) (int, error)
• Write(p []byte) (int, error)
• Close() error

Although those operations would also need Contexts or options so that each read write can exit on cancellation/timeouts.