ipa: Define a plain C API

The C++ objects that are expected to convey data through the IPA API
will have associated methods that would require IPAs to link to
libcamera. Even though the libcamera license allows this, suppliers of
closed-source IPAs may have a different interpretation. To ease their
mind and clearly separate vendor code and libcamera code, define a plain
C IPA API. The corresponding C objects are stored in plain C structures
or have their binary format documented, removing the need for linking to
libcamera code on the IPA side.

The C API adds three new C structures, ipa_context, ipa_context_ops and
ipa_callback_ops. The ipa_context_ops and ipa_callback_ops contain
function pointers for all the IPA interface methods and signals,
respectively. The ipa_context represents a context of operation for the
IPA, and is passed to the IPA oparations. The IPAInterface class is
retained as it is easier to use than a plain C API for pipeline
handlers, and wrappers will be developed to translate between the C and
C++ APIs.

Switching to the C API internally will be done in a second step.

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

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