Carousel: Have Option to Show Full Caption

### Impacted plugin

Jetpack

### What

Prior to the gallery refactoring, the carousel would show the entire caption, even if it was long. The refactor hides everything behind the info button.

### How

We could add a toggle allowing users to display the entire text of the caption if they so wish, so it doesn't get cut off with an ellipses.

Conversely, another approach might be to show the additional information hidden behind (i) by default instead of having to open it for each image.

The issue identifies Jetpack's carousel and the gallery refactoring but names no file or test. Start by locating the carousel implementation and its caption and info-button behavior, then clarify whether the desired change is a full-caption toggle or expanded information by default. Done means the chosen behavior is implemented and verified.

Written by the indexing model from the issue text.