lavu/hwcontext_qsv: add loader field to AVQSVDeviceContext

In oneVPL, a valid mfxLoader handle is needed when creating mfx session
for decoding, encoding and processing[1], so add loader field to
AVQSVDeviceContext. User should fill this field before calling
av_hwdevice_ctx_init() if using oneVPL

This is in preparation for oneVPL support

[1]https://spec.oneapi.io/versions/latest/elements/oneVPL/source/programming_guide/VPL_prg_session.html#onevpl-dispatcher

libavutil/hwcontext_qsv.h

@@ -34,6 +34,17 @@
  */
 typedef struct AVQSVDeviceContext {
     mfxSession session;
+    /**
+     * The mfxLoader handle used for mfxSession creation
+     *
+     * This field is only available for oneVPL user. For non-oneVPL user, this
+     * field must be set to NULL.
+     *
+     * Filled by the user before calling av_hwdevice_ctx_init() and should be
+     * cast to mfxLoader handle. Deallocating the AVHWDeviceContext will always
+     * release this interface.
+     */
+    void *loader;
 } AVQSVDeviceContext;
 
 /**