Project Browser's block settings config schema should validate that the configured sort option is valid

Problem/Motivation

As of ✨ Allow the frontend code to be initialized with a preconfigured query Active , Project Browser blocks have configuration options. One of those options is default_sort, which needs to refer to a sort option defined by the source plugin. At the moment, there's no way to validate that it's a legitimate sort option.

Proposed resolution

Add a custom validation constraint (or find a way to re-use an existing one) which confirms that the configured sort option is, in fact, defined by the source plugin used by the block.