Add cache support for responses without `cache-control: public`

[mcollina]

According to https://www.rfc-editor.org/rfc/rfc9111.html#response.cacheability, it's possible to cache responses if Cache-Control: public is not present.

Going through that list (non exhaustively):

• Support caching data with Cache-Control: private, behind an option (disabled by default)
• Support Expires header
• Caching responses that are not explicitly uncacheable, behind an option (disabled by default)

On that latter point:

Note that, in normal operation, some caches will not store a response that has neither a cache validator nor an explicit expiration time, as such responses are not usually useful to store. However, caches are not prohibited from storing such responses.

This might be useful for some application.