Only validate config which is fully validatable

Problem/Motivation

We discovered that we made an mistake in #3401867: Validate config that gets imported from modules or the recipe's config directory and #3401723: Config modified by action plugins should be validated after it is saved - we validate all config the recipe touches, but not all config is actually validatable. Core ships a fair amount of config that is invalid out of the box (for example, any simple config without a langcode property would run afoul of our validation).

This has the sad effect of making the recipe system unusable in many situations. Since recipes are being used in the wild, this is breaking actual users and sites, so this issue is critical.

Proposed resolution

Core's solution for this is to only validate config that has the FullyValidatable constraint. We should follow core's lead, and only validate config which has that constraint.