General: Add allowed_blocks field to block registration and REST API

[jsnajdr]

There is a new block.json field called allowedBlocks, added in Gutenberg in WordPress/gutenberg#58262. This is a companion patch to support this new field also on the server. It ensures that the following works:

1. If the allowedBlocks field is defined in block's block.json, the objects registered in WP_Block_Type_Registry will have the allowed_blocks field.
2. The allowed_blocks field is returned by the /wp/v2/block-types REST endpoint.
3. The allowedBlocks field will be included in the boostrapped block registration data in the unstable__bootstrapServerSideBlockDefinitions inline script.

Trac ticket: https://core.trac.wordpress.org/ticket/60403

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[github-actions]

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[gziolo]

allowed_blocks should be also explicitly defined in WP_Block_Type class.

[jsnajdr]

allowed_blocks should be also explicitly defined in WP_Block_Type class.

Done ✅

[gziolo]

CI is unhappy about something. Other than that, I have enough to commit it tomorrow.

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Props: jsnajdr, gziolo.

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[gziolo]

Committed with https://core.trac.wordpress.org/changeset/57521.