[RFC] Add support for lightweight post-processing of feature data

[jerluc]

I'd like to propose that we provide a mechanism for allowing some amount of post-processing of feature data to support some common tasks, e.g.:

• Shape simplification or other geometry edits (note that currently simplification is enabled as a server-wide flag with not additional options)
• Property data edits or potentially conditional adding/removing of data
  • String formatting
  • Numerical conversions
  • Adding style information to feature properties
• Creating new features based on tile-wide data
  • Aggregated grid cells
  • Tile-local clusters

The reason to consider this a post-processing step is so that a singular representation of this logic can apply to feature data coming from multiple backend sources, especially for the scenario of computing some tile-wide aggregations.

One thought would be to introduce support for some basic, embeddable scripting language (e.g. Lua, JS, etc.) that can be provided either statically in the tilenol configuration, or potentially in the request URL itself (e.g. script=<BLAH>). By using a simple scripting language (especially a familiar one), users would be able to quickly pick up the scripting capabilities. Furthermore, by providing the option to support request-time scripting, this could be used for very powerful, expressive, and interactive geospatial analyses over mixed datasets.