Use `ExternalVideoInterface` on `LightningPoseConverter`

[h-mayorquin]

Coming from #1589

This looks good. Could you write an issue for LightningPoseConverter to use ExternalVideoInterface and/or InternalVideoInterface? I don't think we should just leave a private legacy video interface hanging around forever.

Originally posted by @pauladkisson in #1589 (review)

[h-mayorquin]

@pauladkisson can you give this a try? The reason I was afraid of changing it myself is the metadata and I think you will be able to go through that faster. Maybe we could try the copilot bot for this.