Add support for renderer objects and related symbol, colorRamp, and classification objects

[elipousson]

Adding support for renderer objects could help support features like R-ArcGIS/arcgislayers#119 and R-ArcGIS/arcgislayers#106 from {arcgislayers}. Here are the three types of "renderers" from the developer documentation:

• Simple renderer
• Unique value renderer
• Class breaks renderer

This will also include adding support for symbol objects:

• Color
• Simple Marker Symbol
• Simple Line Symbol
• Simple Fill Symbol
• Picture Marker Symbol
• Picture Fill Symbol
• Text Symbol

Also color ramp objects:

• Algorithmic
• Multipart

And, finally, classification objects:

• Class Breaks
• Unique Value

The documentation on drawingInfo and renderer in the Web Map Specification docs is another relevant reference.

I've put all these object types in as a checklist so I can check them off when I get them added in. If there are any existing functions for special handling of layer metadata that I should use as a model for structuring these additions, please let me know.

[elipousson]

I just ran across the ESRI CIM Spec documentation and realized that some interoperability with this spec and stylx files from ArcGIS Pro could be a really nice feature https://github.com/Esri/cim-spec