INTPYTHON-624 Add PolymorphicEmbeddedModelField

[timgraham]

todo:

• validation for clashing model fields
• topic guide documentation
• make the field ignored in forms

[WaVEV]

what if multiple submodes has the same field name?

[timgraham]

I called this out in the design doc. I think we'll have to use the system check framework to enforce that common field names at least use the same type, otherwise, we won't know how to prepare the lookup value (e.g. the DatabaseOperations.adapt_<field>_value() methods).

Thinking about it some more, there is also the possibility that nested embedded documents share a field name. In that case, we won't know which field to traverse for the nested lookups that follow.