Gif/Giphy Block: Add Alt Text field

### What

Right now the Giphy block doesn't allow you to set alt-text anywhere, and it seems to pull alt-text from the image on giphy itself. Alt text should be contextual so this seems unexpected and is sometimes very lacking, and this should have an option for people to add their own alt-text related to their content.

### Why

This is an accessibility issue, all images and gifs should have proper alt-text, and with this block we can't set it.

### How

I am not sure how to do this when the whole thing is set in an iframe embed, but I do see alt-text on the original images and in the HTML

Start by locating the Giphy block and tracing how its iframe embed and image alt text are represented. Confirm where a user-provided alt-text value can be entered and rendered, then verify that the block exposes contextual alt text and that existing embeds continue to work.

Written by the indexing model from the issue text.