Event cache status

[pi0]

(context: discussion with @atinux on discord and initial draft of #2519 for the idea of having a way to know if a request event is within the cache and transform it to the client state)

Currently, there is no straight-forward way to know if an event object is within a cachedEventHandler context. It can be determined by checking filtered headers for example like User-Agent header.

One simple way (as initially proposed in #2519) is to add a new event.context.isCached flag to determine this but this looses some context:

• Are we responding first time to a cache or refreshing the cache
• Is the cache skipped
• What headers would be finally allowed to the client (some utils might need to make sure they work as expected)

Because of this, i am thinking to introduce a new event.context.cache: {} namespace to preserve the state:

Keys:

• (if context.cache key exists, it is synonym of context.isCached)
• cache.options: Cache options (should not be modified)

In next steps, we can provide additional info but requires some refactors on implementation:

• cache.isBypassed
• cache.key: Generated internal cache key
• [...]

[pi0]

Landed with #2519. Closing issue for now because most of next steps need complete refactor on cache layer (upcoming).