libcamera: Add a base class to implement the d-pointer design pattern

The d-pointer design patterns helps creating public classes that can be
extended without breaking their ABI. To facilitate usage of the pattern
in libcamera, create a base Extensible class with associated macros.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

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