The config_save action should be able to treat its value as a partial config entity

Problem/Motivation

The "Save config" action treats its incoming value as a literal (albeit token-replaced) array of config data. This is a problem if you want the action to create a config entity, since the entity system is presumably bypassed. It's also a UX problem, because you need to provide the entire config structure to ECA, rather than just the parts you care about.

Proposed resolution

The action should be smart enough to detect if the config it's about to create is a config entity (it can figure this out by the config name prefix), and if so, it should interpret the value to set as a YAML array and pass it to Entity::create() or Entity::set(), depending on whether the entity already exists.