Add --show-changelog flag to flatpak-info

[TingPing]

This shows the latest changelog entry from the appstream data for a given app.

It looks something like this, with italics and lists:

TODO:

• Maybe tweak formatting/wrapping like the other printed strings
• More helpful error messages
• Put changelog in deploy data

Closes #5561

[TingPing]

Hmm, my first take at this was going to add the whole changelog entry to the deploy data which I found a bit clunky. Now thinking about it that probably still makes sense since the appstream data can mismatch the deployed version.