Cannot globally override DataFetcher's on custom operations/fields/properties

[jamesdh]

Overriding DataFetcher's currently is only supported on the automatically provided operations, and is not possible on custom operations or fields/properties.

This is cumbersome when dealing with a large object graph where a given entity type might be an associated property in numerous places, and where you want to restrict the data returned for that type, but not have to supply a custom DataFetcher everywhere the type is used.