Premium content: use new context API server-side

[artpi]

Currently, Premium Content block uses convoluted methods of passing data between the container block and inner logged-out view

The new usage of context could help:
WordPress/gutenberg#21467

Usage of recursion:
https://github.com/WordPress/gutenberg/blob/master/packages/block-library/src/navigation/index.php

[artpi]

CC @eoigal

[artpi]

The terrible solution we want to replace is https://github.com/Automattic/wp-calypso/blob/master/apps/full-site-editing/full-site-editing-plugin/premium-content/premium-content.php#L154

[github-actions]

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