7.0.0 - Introduce versioned ExposableType API

A single JSON-API type can now be exposed under multiple API versions
side-by-side. The internal array-based ExposableTypeMap configuration
is replaced by a typed ExposableType value object, named after the
ExposeType annotation already present in the AnnotationGenerics package.

BREAKING CHANGE: ExposableTypeMapInterface has been redesigned around
the new ExposableType value object and an explicit $apiVersion
parameter:

• getType() -> getExposableTypeByClassIdentifier()
• getClassName() -> getExposableTypeByTypeName($name, $apiVersion)
  -> getExposableTypeByVersionedTypeName()
• getClassNameForProperty() -> getPropertyType($name, $apiVersion, $property)

New: static ExposableTypeMapInterface::forceApiVersion() pins the
active version inside a callable, and the NEXT_VERSION = 'next'
constant marks types belonging to an unreleased version.

ResourceInformationInterface has been tightened with full PHP return
and parameter types; existing implementations need to add matching
signatures.