Evaluate support for non primitive objects

[omry]

This is opening a very large surface area and it will make supporting some things like serialization impossible in such cases, but it's still a highly requested feature.
A few thoughts:

1. For regular DictConfig it could be off by default and enable via a config flag.
2. In Structured Configs, we could support it as a new node type.

[omry]

As a temporary solution, I added an unofficial API in #384 and #385, this is a new flag with the name "allow_objects".

Usage is something like:

    iv = object()
    with flag_override(c, "allow_objects", True):
        c.a.b = iv
        assert c.a.b == iv

Note that this is not yet an official API.
Trying to serialize a config with arbitrary objects is likely to break break serialization (yaml ot pickly) and possibly cause other issues. If you use it please report any issues.