What?

This PR adds a new filter allowing plugins to register inner blocks for <Navigation/>.

Why?

Fixes #31387.

With the current implementation, plugins can still register Navigation inner blocks by listing core/navigation as a parent block. This workaround has a limitation described in #31387 (comment).

How?

By using a filter, blocks can be added to <Navigation> but can still be the top-level blocks.

Testing Instructions

1. From a plugin, add a block to the allowed list using the new filter:

addFilter(
	'blocks.navigation.additionalAllowedBlocks',
	'woocommerce/mini-cart',
	( allowedBlocks ) => {
		return [ ...allowedBlocks, 'woocommerce/mini-cart' ];
	}
);

2. Edit header template part which contains the Navigation block.
3. Try adding an inner block.
4. See the block added above in the block inserter.
5. Can insert that block to the <Navigation>.

Screenshots or screencast