Add support for allowedBlocks attribute in block variations

[stevencamilleri-g2m]

What problem does this address?

Currently there is no way of determining the allowed block types for a variation of a parent block (i.e. limiting the allowed blocks for a Navigation block variation to only support core/navigation-link).

What is your proposed solution?

Add support via an allowedBlocks attribute (like the group block has) to allow developers to define the supported block types on each instance, making it also changeable per instance. This would be more reliable, as the allowed block types would be stored throughout a block variation instance’s life.

[talldan]

Here's a link to the related conversation in slack - https://wordpress.slack.com/archives/C02QB2JS7/p1727941224001089. (it requires a wordpress.org account to access it)