Add transformation ability

[efbenson]

It would be nice to not have to fork a provider if you wanted to append or transform data. For example, if my provider is the AGOL provider and I want to take a layer's results from AGOL on the fly and enrich the data (change some values or add additional fields) it would nice to have a place in the koop layer that exposes the results of the provider with a way for you to mutate it before its sent out vs having to extend the provider for every use case.

Ideally there would be an option for the winnow query to be run against the transformed dataset to apply the query against the transformed data (to filter it further)

[haoliangyu]

You can try v3.17.0 for the new provider options now:

• before for the request parameter pre-processing
• after for the request response post-process

// supply the options when registering the provider
koop.register(myProvider, {
  before: (req, callback) => {
    // do something with the req
    callback()
  },
  after: (req, data, callback) => {
    // tranform the data
    callback(null, data)
  }
})

See #98 for more details.