feature request: override default ffmpeg user agent("Lavf/<version>")

[dongjuanyong]

I opened an issue in clsid2's MPC-HC ( https://github.com/clsid2/mpc-hc/issues/91 ) about some sites blocking accesses with the default user agent used by MPC-HC. Then he told me that it was LAV Splitter which using UAs like "Lavf/58.26.101" and I found this was inherited from upstream project ffmpeg. Since ffmpeg provided an option "user_agent" to override the UA header when calling http request, is it possible to have an option in LAV Splitter to customize the UA? I found clsid2 talked about this some time before. ( https://github.com/Nevcairiel/LAVFilters/issues/199#issuecomment-381939412 )

[lifegpc]

Add support to pass custom http headers is very helpful for play some video links. clsid2/mpc-hc#564

[ale5000-git]

It would be nice if there was a custom interface where the player can pass User-Agent to LAV Filters per-video (not globally).

[iG8R]

@Nevcairiel Please, consider implementing this feature. Thank you.

[NothingCtrl]

Please consider support agent param in .m3u file, example with VLC: #EXTVLCOPT::http-user-agent="..."