Support for caching

Other vulnerability APIs such as PyPI's JSON API provide support for [caching](https://warehouse.pypa.io/api-reference/index.html#caching), allowing the client to avoid making multiple identical requests via [ETag headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag).

The OSV API's use of `POST` requests makes this somewhat challenging, as `POST` responses are not usually cached.

How should OSV support caching, and how should OSV clients cache responses?