Add the endpoint configuration entity

Problem/Motivation

What the generator emits as a plugin class has to be described before it is written: which kind of resource, over which bundle, at which path, with which structure, with which sort order and which listing source. That description is also what the change queue diffs and what a site's compatibility profile pins.

Proposed resolution

• A myrest_endpoint configuration entity carrying the kind, the entity type and bundle, the structure id, the path, the plugin id, and the kind-specific settings.
• Validation at save: the structure exists, the path does not collide, the plugin id is unique.
• A flag marking whether the endpoint is generated or handwritten, so the generator knows what it owns.
• Enabled and disabled state, because switching an endpoint off must not require deleting its description.

Remaining tasks

Everything.