pymapget: add read/iteration bindings for feature model

[MisterGC]

The Python bindings (pymapget) are currently write-oriented, lacking read and iteration support needed for data analysis workflows.

Add read bindings for:

• TileFeatureLayer: __len__, __getitem__ for feature iteration
• Object: __len__, __getitem__ by key
• Array: __len__, __getitem__ by index
• Geometry: geom_type(), num_points(), point_at(), name(), length(), __len__, __getitem__
• GeometryCollection: __len__, __iter__
• AttributeLayer / AttributeLayerList: __iter__
• Feature: id() returning FeatureId
• FeatureId: to_string(), type_id()
• ModelNode base: to_json()
• Fix Feature.to_json() to return string instead of raw nlohmann::json