Block Editor: Enable entity-synced inner blocks.

[epiqueras]

Description

This PR has changes that will be required by any block that needs to sync its serialized inner blocks to one of its entity properties.

E.g. Post Content.

The changes are as follows:

• Add a controlled props (value/change handlers) mode to InnerBlocks so that we can sync other variables with it.
• Make the entity save hook save freshly selected values to avoid race conditions when making "function edits". I.e. when we set a property to a function that serializes blocks to avoid serializing on every change. This change will also just make the hook more resilient under any conditions.
• Export the serialization utility from the editor package so that blocks can use it.

How has this been tested?

These changes were extracted from #17263, where they were verified to work. They also should not break any existing functionality and will facilitate the parallel development of most full site editing blocks like Post Content and Template Part.

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. .

[epiqueras]

This was merged as part of #19203.