Custom Templates: Use "title" from the theme.json

[Mamaduka]

Description

Updates _gutenberg_get_template_files method to merge customTemplates definition from the "theme.json."

PR introduces new _gutenberg_add_template_info helper method.

Fixes #32818.

How has this been tested?

1. Install and Activate TT1 Blocks.
2. Create a page.
3. Templates dropdown should display a title - "Page without title" instead of a "slug."

Screenshots

Before	After

Types of changes

Bugfix

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• 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 (please manually search all *.native.js files for terms that need renaming or removal).

[youknowriad]

Works well 👍

[Mamaduka]

Thanks for the review, @youknowriad.