Block Templates: Fix PHP notices on WP 5.8

[Mamaduka]

Description

When the plugin runs on WP 5.8.x, it will use the WP_Block_Template class from the core, which doesn't have new properties. To avoid PHP notices, we've to use dynamic properties when building templates from the file.

Note: The issue doesn't affect WP 5.9.

How has this been tested?

1. Clone this branch.
2. Open Site Editor with WP_DEBUG_LOG enabled.
3. The debug.log shouldn't contain notices.

Log

PHP Notice:  Undefined property: WP_Block_Template::$origin in ~/Projects/gutenberg/wp-content/plugins/gutenberg/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php on line 467
PHP Notice:  Undefined property: WP_Block_Template::$author in ~/Projects/gutenberg/wp-content/plugins/gutenberg/lib/compat/wordpress-5.9/class-gutenberg-rest-templates-controller.php on line 477

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).

[cbravobernal]

Hi there!

I'm still having those notices when being on site editor.

Attached a Loom Video:
https://www.loom.com/share/8b7dd24c887e4dd1bf57089112a9970b

[Mamaduka]

Thanks for testing, @c4rl0sbr4v0.

Are you using wp-env with WP 5.8.x?

I've to call it a day, but will check back on this issue tomorrow morning :)

[cbravobernal]

I was using wp-env with a development version (5.9-alpha-52268).

Using 5.8 is working fine!

[Mamaduka]

Makes sense :) This should fix the notice for 5.9 - WordPress/wordpress-develop#1967.