Block Editor Tools: revisit registration and directory structure

Following-up from this comment and the conversation above:
https://github.com/Automattic/jetpack/pull/16746#issuecomment-672771493

> As we extend the type of editor tools we have in Jetpack (blocks, extended core blocks, sidebar plugins), I think it would be worth revisiting our directory structure in `/extensions`, as well as our registration tools.
>
> Maybe we could have `jetpack_register_block`, `jetpack_extend_block`, and `jetpack_register_plugin`, all handling plan gating behind the scenes, and have a matching directory structure like:
>
```
extensions
|
`- shared -- for utilties
- blocks -- for blocks created by Jetpack
- plugins -- for sidebar plugins like Social Previews, shortlinks, Publicize, ...
- extended-blocks -- for things like the VideoPress feature, the cover block extension, ...
```
>
> This is not a small refactor, but I think it would be worth it to avoid further confusion in future work, by all the teams that contribute blocks to Jetpack.

cc @getdave @marekhrabe @retrofox @pablinos

This feels even more relevant now that we're working on updating the Paid flows (#16611).

Start by reviewing the existing /extensions directory and the registration tools discussed in the linked pull-request comment. Determine how blocks, extended blocks, sidebar plugins, shared utilities, and plan gating are currently organized; done would mean an agreed directory structure and registration approach that avoids confusion for future Jetpack contributors.

Written by the indexing model from the issue text.