Image Compare: add caption option to each image

#### Is your feature request related to a problem? Please describe.

Sometimes when comparing images the differences are subtle, so adding a text in the bottom corner would be useful to let a reader know what they should see.

#### Describe the solution you'd like

An option to add text to both images. It would be great to be able to let the poster add the text instead of having one predefined. Useful for example in multilingual sites. Something like this: https://i.imgur.com/gu1u7bx.png

#### Describe alternatives you've considered

There are a few plugins I've tried, but they all have issues with the Lazy Images module: https://github.com/Automattic/jetpack/issues/9929
Adding the text feature would solve the above issue in a way.

Locate the Image Compare entry point and inspect how it interacts with the Lazy Images module before deciding where the option belongs. Reproduce the existing comparison behavior, then verify that poster-supplied text can be shown independently on both images without breaking lazy loading.

Written by the indexing model from the issue text.