Enable local builds with FFmpeg8

[Dan-Flores]

This PR adds 8 as an option in load_torchcodec_shared_libraries() and _core/CMakeLists.txt to enable FFmpeg8 builds.

In addition, to enable local builds with FFmpeg8, we need to update our pixel_format setting approach.
Old flow (<= FFmpeg7) :

• Create and init the filter with avfilter_graph_create_filter()
• then set "pix_fmts" with av_opt_set_int_list .

New flow (>= FFmpeg8) :

• Allocate the filter with avfilter_graph_alloc_filter
• set "pixel_formats" with av_opt_set
• then init with avfilter_init_str.

These changes are encapsulated FFmpegCommon.h in createBuffersinkFilter(), which either returns AVFilterContext* or nullptr if an error occurred.