IDatastoreFields

Restrict changes or override fields passed to [datastore_create](http://docs.ckan.org/en/latest/maintaining/datastore.html?highlight=datastore#ckanext.datastore.logic.action.datastore_create)

This interface would let me restrict the datastore field definitions for datasets with dataset types owned by [ckanext-recombinant](https://github.com/open-data/ckanext-recombinant). Recombinant requires each organizations' datastore table have exactly the same columns so they can be combined into a single output.

It would allow an extension to be built that would enforce a [Tabular Data Package field schema](http://data.okfn.org/doc/tabular-data-package) for specific dataset resources. The Tabular Data Package schema could be referenced, stored as a resource or defined within the dataset metadata.

It would allow extensions to be built that store controlled lists as datastore tables. It shouldn't be possible for users to modify the fields in a controlled list.

Start with the datastore_create action in the linked CKAN documentation and determine where field definitions can be restricted or overridden. Done should let extensions enforce consistent columns for selected dataset types or resources and prevent users from changing fields in controlled-list tables.

Written by the indexing model from the issue text.