Add the MyrestFieldShape plugin type

Problem/Motivation

The engine's core idea is that a field type plus a choice of shape produces a fragment of JSON, and that this mapping is a plugin rather than a branch in a helper. Today the same job is done by thousands of lines of methods on a helper class, four times over across the forks.

Proposed resolution

• Plugin type MyrestFieldShape with a manager, an interface, a base class and an attribute-based discovery.
• The interface takes an entity, a field definition and the property settings from the structure, and returns an array fragment plus cache metadata — cacheability travels with the fragment, not around it.
• Each plugin declares which field types it can serve, so the planner can propose one and reject an impossible pairing.
• A shape may be asked for a value that is empty; the contract says what it returns then, once, rather than each plugin deciding.
• Unit and kernel tests for the manager, the base class and the contract.

Remaining tasks

Everything. Every issue that follows in this stage is one plugin against this contract.