Implement shared stub dependencies for shared FFmpegHEADdev

* The idea is to make users able run the FFmpeg plugin without
  additional third-party installations, namely OpenSSL and VAAPI.
  It has been working with static linkage, the current patch
  implements it for shared linkage.
* In a nutshell, we compile libs libQt6FFmpegStub-somedep.so.x
  and make FFmpeg dependent on the instead original libsomedep.so.x.
  Users might trivially replace the shipped FFmpeg with thier one
  without stubs just by deleting/changing the files.
* The solution is scalable on Android and other libs.
* Patching of the FFmpeg dependencies we do in the related
  patch codereview.qt-project.org/c/qt/qt5/+/550318
* More information and discussions are in the ticket
  and the discussion page intranet.qt.io/display/QTRD/Shipping+of+shared+FFmpeg+with+dependencies

Pick-to: 6.7 6.5
Change-Id: I322641abca4a1010e65f74a0a8265b824ceab031
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Jøger Hansegård <joger.hansegard@qt.io>

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