VideoPress Feature Request: Enable subtitles or captions by default

### Impacted plugin

Jetpack, VideoPress

### What

Have the option on VideoPress blocks to enable or show captions/subtitles by default.

### How

Requested in 9061123-zd-a8c, the user argues it would be a good addition to the VideoPress block to allow users the option to show subtitles or captions automatically. Right now, a video can be set to autoplay, but a visitor still needs to click and enable captions from the player.

I didn't see any existing feature requests for this, so I'm creating one here. Apologies if there's an existing one I missed. :)

Start at the VideoPress block and its player settings, comparing the existing autoplay control with how visitors currently enable captions or subtitles. Define the expected default behavior and option for blocks, then verify that captions appear automatically when configured and that existing autoplay behavior remains unchanged.

Written by the indexing model from the issue text.