libcamera: add IPA proxy

Add an IPAProxy class whose implementations will act as a proxy between a
pipeline handler and an isolated IPA interface. Also add an IPAProxyFactory
that will construct the IPAProxy implementations as necessary.

Update Doxygen to ignore the directory where IPAProxy implementations will
reside.

Signed-off-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

test/libtest/test.cpp

@@ -25,6 +25,10 @@ int Test::execute()
 	if (ret)
 		return errno;
 
+	ret = setenv("LIBCAMERA_IPA_PROXY_PATH", "src/libcamera/proxy/worker", 1);
+	if (ret)
+		return errno;
+
 	ret = init();
 	if (ret)
 		return ret;