Support RFC 5861 Cache-Control Extensions

[ItsVeryWindy]

Is there an existing issue for this?

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Is your feature request related to a problem? Please describe the problem.

[https://httpwg.org/specs/rfc5861.html](RFC 5861) extends the Cache-Control header to include two new directives for http responses, stale-while-revalidate and stale-if-error.

stale-while-revalidate in particular is supported by all main browsers, stale-if-error whilst doesn't appear to have browser support is in use by some cdns.

Currently to set either of these means having to manually play about with the Cache-Control header and/or implementing your own middleware.

Describe the solution you'd like

It would be nice if either of these could be supported by the ResponseCachingMiddleware in order to signal clients how caching should be handled.

Additional context

No response