[feat] Add ability to define properties on Container subclasses

[alberttorosyan]

Support for ORM-like properties definition:

class Run(Container):
  name = Property()
  archived = Property(default=False)

would result to creation of name and archived attributes for Run instances, as well as handle serialization/deserialization in rocksdb.

On the UI SDK side, while parsing the API call result stream, a special keyword $property is used to determine the list of properties. These properties can be accessed as attributes, similar to the Python SDK.

[mihran113]

left some minor comments, other than that looks great