Video Block: add the ability to edit video thumbnail poster images

[designsimply]

The option to "Select Poster Image" was requested by someone via support:

I mostly use the mobile app and I upload a lot of videos. Within the Wordpress iOS app, I have yet to find a way to select a video thumbnail (select a poster image). It’s only one simple function but since I need it so regularly, I miss not having it. Otherwise, I’m very pleased with WordPress! Great work!

^{(internal references: p4a5px-2st-p2, 2444886-zen, @SarahFG)}

[designsimply]

For reference, here are some screenshots of the current video block from 13.5 beta testing. Tapping the pencil icon when the video block is selected just opens an option to replace the video.

^{Tested with WPAndroid 13.5-rc-1 on Pixel 3 Android 10 and WPiOS 13.5.0.0 beta (TestFlight) on iPhone 6S iOS 13.2.}

Both WP Admin in WordPress 5.2.4 and Gutenberg Calypso have the option to "Select Poster Image".

^{Tested with `exotic-rockhopper.jurassic.ninja`self-hosted site running WP 5.2.4 and no plugins using Chrome 77.0.3865.120 on macOS 10.14.6.}

^{Tested with `aboutmakingmovies.movie.blog` WordPress Premium site on Firefox 69.0.1 on macOS 10.14.6.}