Add support for enterprise extension

[tkrille]

@bng-okta suggested, that supporting the well-defined enterprise extension is a viable alternative to supporting arbitrary complex fields in extension.

First Question is: Do we want to add this feature and for what release should it initially be scheduled? I've added it to the v3.1 milestone for now.

[wallner]

I agree. I think we should schedule it for 3.1

[tkrille]

I don't like the idea of a special treatment, but I think in the end this feature is worth the trade-off. So, yes, let's try it. Beware, that this also impacts the connector4java.