WordPress App: Please add Custom HTML Block on the app

### What

The title is pretty much it :D A user reached out on chat, informed us that they like using our app (WP for Android,) but they found out that they need to open WordPress.com on browser to enter affiliate links/Amazon links (HTML iframe code) on their blog post. It works, but it's not ideal as they prefer to do it on app (as app is faster on mobile/handheld gadget.)

Currently, we have these blocks on our app:
https://apps.wordpress.com/support/mobile/posts-and-pages/list-of-available-blocks/

Maybe we can add a couple more, Custom HTML Block included, to make things awesomer?

### Why

Opening WordPress.com editor on a browser on tablet or handphone is not an ideal situation -- for some reason, it can go quite slow and laggy (my personal experience) 🤔 That's why folks opt to use app for their WordPress.com website needs when they need to use their phone or tablet.

Now, building websites can be from writing a post to embedding affiliate links. It would be awesome if we make it easier for our users and customers to be able to add HTML links on their posts using Custom HTML Block.

### How

_No response_

Start with the app's current block list at the linked WordPress mobile support page and compare it with the requested Custom HTML Block. Determine where the mobile post editor defines available blocks, then verify that users can add affiliate or iframe HTML in an app post and that the block is represented correctly when editing or saving.

Written by the indexing model from the issue text.