Configure nested types as complex types by default

[roji]

When mapping a complex type hierarchy, the user must currently manually configure each and every non-primitive property as complex; we should consider doing this by convention the moment the top-most containing property is configured as complex.

This is similar to the situation with JSON and owned (one of these must be manually specified for the entire hierarchy). As discussed, I'm assuming that even when we enable regular navigations from complex types out of the complex hierarchy, we'd still not want that to be the default.

[AndriySvyryd]

Currently, once something is marked as a complex type then all references to it will be configured as complex properties. This means that with the above change everything transitively referenced from a complex type will be considered a complex type. For some models this could result in every type except the ones explicitly mapped as entity types to become complex types. We should discuss whether this is the desired behavior.

[roji]

Note #31344, which is about removing the behavior that a CLR type is configured as a complex type if it's configure elsewhere in the model as a complex type. This should take care of the above objection.