precompute_online: feature names are about 80% of every vector blob #7075

Every pre-computed vector repeats the service's full feature name list, and for a wide service that is most of the blob. For a 90-feature service on Feast 0.67.0:

So the single-read design moves 4x the bytes of the per-view path it replaces. Redis time per 500-entity read went from 2.0 ms to 0.4 ms with the single read, but the client spends it back parsing names.

Proposal, in #7077: an opt-in precompute_omit_names on FeatureService. Vectors written for such a service carry values, per-view timestamps, and a fingerprint of the service's ordered names. The fast path validates a nameless vector by fingerprint and takes names from the service; vectors with names keep working. Same service: 834 B per entity, 414 KB per 500-entity read. Needs one field on FeatureServiceSpec and one on PrecomputedFeatureVector.