Site Title: Access the new API endpoint for editing the site title

[scruffian]

Description

This implement the new REST API endpoints added here:
https://core.trac.wordpress.org/ticket/48885

How has this been tested?

Create a new post and add the site title block. You should be able to view and edit the site title.
Now log in as an "Editor". You should be able to view the site title, but not edit it.

Types of changes

New feature (non-breaking change which adds functionality)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR. .

[earnjam]

@scruffian Based on what I read in the Trac ticket, this approach is outdated based on feedback from the REST API team, correct? I think we had decided not to add a new /settings/public route.

I see you have a couple of other PRs related to this open in #18760 and #19967. Should we close this one in favor of those two?

Also should add a note based on the changes in this PR, if we are going to implement something that relies on REST API changes in core, we'd also want to implement those changes in Gutenberg so that the plugin is not dependent on WP core trunk.

[scruffian]

Yeah lets close this since we decided to do it a different way.