injection and JNDI lookup of EntityAgent

[gavinking]

We need a way to expres a dependency on an EntityAgent, analogous to @PersistenceContext. At first I thought we could reuse @PersistenceContext or @PersistenceUnit for this, but actually that doesn't work for the JNDI lookup scenario, which might be important for Jakarta Data repository clients. (In fact, there are other reasons why @PersistenceContext is not quite right.)

So I suppose we might need an @PersistenceAgent annotation. (Blah.)