AssetInterceptor not running for property resolvers

[michaelbromley]

Describe the bug
When an entity has an entity resolver field defined, and that field includes Assets, those Assets are not passing through the AssetInterceptor and therefore the preview and source URLs are not being correctly resolved.

To Reproduce

1. Add an asset to a ProductVariant in the product detail page
2. Save changes
3. Refresh page
4. ProductVariant image is a broken link. Does not include the prefix.

Expected behavior
ProductVariant asset urls should be correctly prefixed.

Environment (please complete the following information):

• @vendure/core version: 0.1.2-beta.12

Additional context
Debugging with breakpoints shows that the sequence is as follows:

1. ProductService.product resolver invoked, returns a Product (without variants)
2. AssetInterceptor.intercept method called on the object returned in 1. And assets present are correctly prefixed.
3. ProductEntityResolver.variants method is called.
4. Response is returned without the variant assets running through the interceptor.

[michaelbromley]

This is a limitation of Nest - see nestjs/graphql#320. The probable solution to this is to convert the AssetInterceptor into an Apollo server plugin, which should get run on the final object just prior to returning the response to the client. This would also potentially improve performance since only the selected subset of fields may be needed to be traversed.