[enh] promote `other_keys`

[dajmcdon]

Can we move other_keys out of the additional_metadata List? This seems like, by far, the most common use case we've seen for this list. And it already has its own special handling in the new_epi_df() constructor.

[brookslogan]

Yeah, we definitely need to get to this, and just remove the other additional_metadata. We don't handle that any better than direct attrs.

Probably a dupe of #186

[brookslogan]

I think this was magicidentally closed.