RSS Block: support className attribute

[Soean]

Description

Adds the custom className attribute to the RSS block.
Also orders the attributes in a alphabetical order.

[codebykat]

I had to rebase this because I was getting "The editor has encountered an unexpected error" for whatever reason. But after a rebase and applying the suggested change by @noisysocks it is working great!

Testing steps I followed:

1. Add an RSS block to a post
2. Add a URL and click "Use URL"
3. Add an Additional CSS Class under Advanced
4. Update the post and verify that the class name was added, along with wp-block-rss
5. Delete the class name, update the post and verify that the class name is set to only wp-block-rss (with no empty spaces in the DOM).

[Soean]

merged in #14533