Add LocalResponseCache filter to Spring Cloud Gateway Server Web MVC

[nikita-kibitkin]

Summary

• Add the route-level LocalResponseCache filter to the Web MVC module, mirroring the cacheability semantics of the WebFlux filter: bodiless GET requests only, 200/206/301 responses, request/response Cache-Control handling, Vary handling, no-cache revalidation, and max-age recalculation on cache hits
• Back the cache with a per-route Caffeine cache (optional dependency, auto-configuration guarded by @ConditionalOnClass) with optional size-based eviction; both filter parameters (timeToLive, size) are optional and default to 5 minutes / unbounded
• Capture the response body at write time through a ServerResponse decorator, so the deferred proxy write path stays intact, composed filters keep working, and the cached body always belongs to the response that produced it; writes that do not carry the full body (304 answers to conditional requests, async and view-rendering responses, statuses mutated after wrapping, framing mismatches) are never cached
• Never cache responses with a streaming media type (spring.cloud.gateway.server.webmvc.streaming-media-types)
• Update the filter documentation and enable the page in the navigation

Builds on the prototype started by @ingogriebsch - his commits are kept in this branch. Ingo, a review from you would be very welcome.

Fixes gh-4182

Notes for reviewers

• Scope is per-route only. The WebFlux global cache and the @ConfigurationProperties defaults are intentionally out of scope for this PR; the docs reflect that.
• Some limitations are inherited from the WebFlux implementation on purpose, to keep the two stacks in sync: forbidden Cache-Control values (private, no-store) are matched per header value rather than per directive, Vary: * is only detected as a standalone value, Set-Cookie headers are cached and replayed, and there is no per-entry size cap. I can file a follow-up issue covering both stacks if that is preferred.
• One deliberate improvement over WebFlux: a conditional request that matches a cached ETag is answered with 304 from the cache (covered by tests).

Testing

• ./mvnw -pl spring-cloud-gateway-server-webmvc -Dtest='ResponseCache*' test (37 unit + 16 integration tests)
• ./mvnw -pl spring-cloud-gateway-server-webmvc test (full module green, including checkstyle)

[spencergibb]

I'm going to have to see how we handle this because you have included commits that are not yours. The earliest this would likely get merged is September or October

[nikita-kibitkin]

The prototype commits are @ingogriebsch's, kept as-is so his authorship is preserved; each of them carries his own Signed-off-by.

@ingogriebsch could you confirm here that you agree to your commits being included in this PR - and approve it if you think it is ready?

If a different shape is easier to handle, tell me and I will rebase - for example squash the branch into a single commit with a Co-authored-by trailer.

[spencergibb]

He didn't submit them to us. I'll get guidance from my team next week.

[ingogriebsch]

@spencergibb I am the one who originally created Issue #4182. I had also created a fork and started working on the ticket. However, since I’m currently very busy and @nikita-kibitkin offered to take over the implementation, we agreed on how to proceed; he took over the codebase I had started and finalized it. The result is the current PR (which I personally haven’t had a chance to review yet). We weren’t aware that this might cause confusion and additional work; please accept our apologies.

I’m still fine with Nikita taking over and continuing my commits. And I’m also fine with the PR being accepted. I hope this information helps your team decide how to proceed with this. Personally, I’d love it if the PR were accepted and the feature became available as soon as possible. :)

[spencergibb]

Yes, thanks all. Based on team feedback, your response and the signed commits, everything is ok from our side.

[spencergibb]

Just a reminder, new features don't go in patch releases and we won't branch for a new minor until probably September or October and ga in November. So it will be a while before this gets merged