Add KEY JOIN syntax to simulate joining using generated foreign keys

[lukaseder]

If two entites are formally related using a foreign key relationship, it is only natural for users to join them:

create.select()
      .from(T_AUTHOR)
      .join(T_BOOK).on(TAuthor.ID.equal(TBook.AUTHOR_ID));

This can become tedious after a while. If foreign key relationships are formally available to jOOQ, then any of these "virtual" syntaxes could be imagined:

// Let the user assume that the key exists.
// Throw exceptions if there is no key in T_BOOK referencing 
// T_AUTHOR (the order is important), or if keys are ambiguous
// Additional conditions can be added to the generated ON clause
create.select()
      .from(T_AUTHOR)
      .join(T_BOOK)
      .onKey()
     [.and() | .or() | ...]

// Let the user provide a explicitly.
// This will throw an exception if the provided key does not
// link T_BOOK to T_AUTHOR or vice versa (the order isn't important)
// Additional conditions can be added to the generated ON clause
create.select()
      .from(T_AUTHOR)
      .join(T_BOOK)
      .onKey(Keys.FK_1)
     [.and() | .or() | ...]

// Or let them provide fields that participate in a key
// This will throw an exception if the provided fields do not
// fully participate in a key that links T_BOOK to T_AUTHOR or vice
// versa (the order isn't important)
// Additional conditions can be added to the generated ON clause
create.select()
      .from(T_AUTHOR)
      .join(T_BOOK)
      .onKey(TBook.AUTHOR_ID, ...)
     [.and() | .or() | ...]