Add string-based access API for RealmObject

[nirinchev]

Adds a string-based API to read properties of a RealmObject. Right now we only support reading as that's required to get migrations working, but it can be extended in the future to also support setting.

The API is the following:

final obj = realm.dynamic.find("Foo", 123);

// Get simple primitive
final str = obj.dynamic.get<String>("stringProp");

// Get another object
final bar = obj.dynamic.get<RealmObject>("bar");
final barString = bar.dynamic.get<String>("stringProp");

// Get a list of objects
final bars = obj.dynamic.getList<RealmObject>("bars");

// Iterate the list as normal
for (final bar in bars) {
  print(bar.dynamic.get<DateTime>("dateProp");
}

Part of #70.

[fealebenpae]

I'm not a super huge fan of hiding this API under a dynamic property like we did in .NET :/

[nirinchev]

The API is kind of large and niche, so I'd rather not pollute the main interface but happy to discuss alternatives.

[nirinchev]

Superseded by #853.