List a template's assets

The template's fields on their own, without the document body beside them. Same `fields` and `layers` as `GET /api/v1/templates/{id}`, but a document can be large and a caller that only needs to know what to put in `data` should not have to download one to find out. Add `?primary=true` for content fields only. A canvas layer carries every typography property it supports, so a five-layer template answers with 56 fields of which four are the ones anyone means. `field_count` reports both totals either way, so a filtered response never looks like the whole picture. `sample_data` is a ready-to-send `data` object built from the fields' own example values, and always contains the primary fields only.