No-Vary-Search: Handling multiple matches and efficient cache implementation

[jeremyroman]

Migrated from https://github.com/WICG/nav-speculation/issues/210.

The most active discussion is at the bottom of that issue -- permitting cache implementations to assume a single non-empty No-Vary-Search field value per path in order to allow simpler & more efficient implementations, and missing if this isn't the case.