Explicitly annotate api

Problem/Motivation

Currently the module has been using the Drupalism regarding API that are not well spelled out.

Steps to reproduce

Proposed resolution

Label classes/methods that are API(expected to be called outside the module) as @api
Any class or method not labeled @api will be subject to breaking changes without BC.

Clarifying: if an entire class is marked @api all methods on the class are also API spec.

This will also place us in a spot that eventually we could allow PHPStan to help implementers become aware of non-api usage.

Remaining tasks

User interface changes

API changes

None, only clarification of what is and is not already API.

Data model changes