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

[elipousson]

Adding support for renderer objects could help support features like https://github.com/R-ArcGIS/arcgislayers/issues/119 and https://github.com/R-ArcGIS/arcgislayers/issues/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