Refine summary stats backend RPCs for data explorer, add Python implementation and basic scaffolding

[wesm]

In short:

• Extracts ColumnSchema.type_display enum into a new top-level ColumnDisplayType enum
• Fixes generate-comms.ts to handle enums in components.schemas
• Deletes some dead (?) type alias code in generate-comms.ts that wasn't needed anywhere that I could tell. I made sure Amalthea followed this
• Added interfaces for each type of summary stats to match what we want to show in the UI
• Added basic implementations for Python
• Did minimal kludge to show the numeric stats in the UI after clicking a bit to get React to re-render (not sure how to handle async calls there)

I concluded that the backend will need to send numeric summary stats as numbers so the UI can normalize formatting, so I can change the numeric summary stats struct to return numbers whenever we are ready to take responsibility for this formatting in the UI. See also related #2614

See downstream posit-dev/ark#293

[wesm]

Merged this to keep things moving so Brian can pick up these changes. Any feedback here I can take into account in the next PR