Add handling for __default attribute to block bindings for pattern overrides

[talldan]

Trac ticket: https://core.trac.wordpress.org/ticket/61333

This is the PHP backport for the gutenberg PR WordPress/gutenberg#60694.

It adds handling for a __default block binding attribute for pattern overrides. When this binding attribute is encountered by the block binding code it dynamically adds support for all supported block binding attributes.

Testing instructions

This has been rebased with the latest package updates, so is now fully testable.

1. Add a heading, paragragph, image and button to a post (or use a pattern that contains those blocks)
2. Select them and choose 'Create pattern' from the block options menu
3. Name the pattern, keep the Synced toggled enabled, and create the pattern
4. Select 'Edit original' from the toolbar of the pattern that was inserted into the post
5. For the heading, paragraph, image and button block, select each one individually, and in the advanced inspector tools choose 'Enable Overrides'. Give the blocks a name in the resulting dialog.
6. Save the pattern and go back to the post
7. Enter some content into the heading, paragraph, image and button with the pattern instance int he post
8. Preview the post and ensure the content is correct on the frontend

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[github-actions]

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Core Committers: Use this line as a base for the props when committing in SVN:

Props talldanwp, petitphp, mukesh27, isabel_brison, kevin940726.

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[tellthemachines]

I can't really speak to the code as I'm not familiar with block bindings logic but this is working well for me in testing, both when creating a pattern from existing post content and creating one from scratch in the site editor.

[tellthemachines]

Typo: "build a binding array"

[talldan]

Thanks, fixed in cbc99b5

[kevin940726]

Works well in my testing!

[tellthemachines]

Committed in r58289.