Add support for associations of type Map, including customizable map key

[haqer1]

The core equivalent from JPA 1 is the following annotation:
https://javaee.github.io/javaee-spec/javadocs/javax/persistence/MapKey.html

P.S.
But a few other "bells-and-whistles" have been added in JPA 2. E.g.:
https://javaee.github.io/javaee-spec/javadocs/javax/persistence/MapKeyEnumerated.html
P.P.S. It's also worth considering here whether to use the same annotation(s), or to add distinct one(s).