Set User Agent for FFmpeg calls

[felipecrs]

Closes #4369

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[felipecrs]

Another option I thought of was to set them here, disallowing users to override:

[NickM-27]

Another option I thought of was to set them here, disallowing users to override:

I see no reason to make it mandatory, users should be able to change it if they want to

[felipecrs]

I see no reason to make it mandatory, users should be able to change it if they want to

Cool, I agree.