feat: allow skipping ffplay/ffprobe on autodownload

[cnaples79]

Summary

• add env flag to autodownload flows
• now skips ffplay/ffprobe when the flag is set
• update examples to pass the new argument

Rationale

• reduces footprint when only ffmpeg is needed by allowing users to drop ffplay/ffprobe during autodownload

Test Plan

• compiled locally

Fixes #93

[nathanbabcock]

Hey @cnaples79 thank you and happy New Year!

This will be a great feature to have and I know a few people have been looking for it.

One minor change I'd request: since unpack_ffmpeg is part of the public API, I'd like to keep it backwards-compatible. Adding a third required parameter would cause compile errors for anyone using that function directly after updating.

My preference would be an additional variant, unpack_ffmpeg_without_extras or something like that, so this change is purely additive. That's how we introduced progress events for downloads recently.

Longer term as we collect more of these options perhaps we'd introduce a config struct people could pass rather than env vars. But that would be in a major version bump and in the meantime people could make use of this right away.

[nathanbabcock]

I believe this is solved in #102. Thank you for getting the ball rolling with this PR