OutputCaching + YARP interoperability

Hello,

As per discussion over in the YARP repo https://github.com/microsoft/reverse-proxy/discussions/1851.

In the scenario where OutputCaching is being used with YARP I would like a way to be able to validate the cached items on request.

I believe there would be value in having a way to make YARP and OutputCaching work together.
Have YARP leverage OutputCaching instead of having OutputCaching on top of YARP.

At a high level I'd like to achieve the following pipeline:

Get request > YARP processes its routes/clusters > YARP/[custom middleware](https://microsoft.github.io/reverse-proxy/articles/middleware.html#custom-proxy-middleware) gets the output cache headers and performs a head request against the origin server with the "If-Modified-Since" and "If-None-Match" headers > return the Output Cache data if valid else back to YARP to process the request per usual and the output is cached

Perhaps something like the HttpContext.Features `HttpContext.GetReverseProxyFeature()` extension method for YARP middleware but for output caching?

EG:
```c#
endpoints.MapReverseProxy(proxyPipeline =>
{
proxyPipeline.Use((context, next) =>
{
var proxyFeature = context.GetReverseProxyFeature();
var outputCacheFeature = context.GetOutputCacheFeature();
var cluster = proxyFeature.Cluster;
var destinations = proxyFeature.AvailableDestinations;
var cachedHeaders = outputCacheFeature.GetHeaders(context.Request);

if (cachedHeaders is not null)
{
// logic to validate and optionally return the cached item
}

return next();
});
proxyPipeline.UseSessionAffinity();
proxyPipeline.UseLoadBalancing();
proxyPipeline.UsePassiveHealthChecks();
});
```

This could also be used by other non YARP things that have access to the HttpContext.

My end game goal is to be able to replace IIS+ARR/Nginx's disk based content caching with YARP and hopefully OutputCaching.
Ideally I wont have to go down the path of writing my own caching mechanisms for YARP as the guts of OutputCaching appears to be solid as.