count exact number of frames with ffprbe -count_frames

[Rusteam]

What changes are proposed in this pull request?

Count exact number of frames in a video.

How is this patch tested? If it is not, please explain why.

added a unit-test to the intensive group.

Release Notes

Is this a user-facing change that should be mentioned in the release notes?

• No. You can skip the rest of this section.
• Yes. Give a description of this change to be included in the release
  notes for FiftyOne users.

Exact number of frames can be counted with fiftyone.utils.video.exact_frame_count().
This method uses -count_frames argument of ffprobe which decodes the entire video to count the frames and can be very slow.

What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
• Other

[brimoor]

Amazing, thanks! 🙏