Metamodel should expose methods to determine *all* JPA-handled types

JPA has always had a Metamodel API to find out about managed entities, managed superclasses and embeddable type. With the introduction of attribute converters, other types get under control of the JPA provider, too. E.g. depending on whether an `AttributeConverter` is registered for it or not, a `LocalDateTime` (or any other type) could be effectively become a JPA understood "primitive" type.

Currently there's no way for clients to use the Metamodel API to find out about that fact that there certain types are JPA-understood ones. It'd be great if that could be found out.
#### Affected Versions
[2.1.1]

Start by reviewing the JPA Metamodel API and the AttributeConverter behavior described in the issue, especially how managed entities, managed superclasses, embeddable types, and converted types are represented. Define what clients must be able to query for all JPA-handled types, then verify that the proposed API covers converted types such as LocalDateTime as well as existing managed types.

Written by the indexing model from the issue text.