libcamera: request: Move metadata_ to Private

Address a long standing \todo item that suggested to implement a
read-only interface for the Request::metadata() accessor and deflect to
the internal implementation for the read-write accessor used by pipeline
handlers.

Signed-off-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Acked-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Daniel Scally <dan.scally@ideasonboard.com>

include/libcamera/request.h

@@ -50,7 +50,7 @@ public:
 	void reuse(ReuseFlag flags = Default);
 
 	ControlList &controls() { return *controls_; }
-	ControlList &metadata() { return *metadata_; }
+	const ControlList &metadata() const;
 	const BufferMap &buffers() const { return bufferMap_; }
 	int addBuffer(const Stream *stream, FrameBuffer *buffer,
 		      std::unique_ptr<Fence> &&fence = {});
@@ -68,7 +68,6 @@ private:
 	LIBCAMERA_DISABLE_COPY(Request)
 
 	ControlList *controls_;
-	ControlList *metadata_;
 	BufferMap bufferMap_;
 
 	const uint64_t cookie_;