Add WebSession-based sticky load balancing filter

[Gebreyesus]

## Add WebSession-based sticky load balancing (server-side, per-route)

This PR adds per-route, server-side session affinity to Spring Cloud Gateway using the gateway's own `WebSession` as the affinity store.

### Problem

`ReactiveLoadBalancerClientFilter` passes a `RequestDataContext` to the load balancer. That context carries HTTP request metadata but no reference to the `ServerWebExchange` or `WebSession`. A custom `ReactorServiceInstanceLoadBalancer` therefore cannot implement server-side session affinity without fully replacing the default filter.

The existing `RequestBasedStickySessionServiceInstanceListSupplier` solves a related but different problem: it keys stickiness off a client-supplied cookie and applies uniformly to a service, with no per-route opt-in.

Our production case involves a legacy monolith that cannot achieve distributed sessions. The gateway itself must track which instance a client is pinned to, stored server-side in `WebSession`, with per-route granularity.

### What this PR adds

**`WebSessionStickyGatewayFilterFactory`** (the recommended opt-in mechanism)

A per-route `GatewayFilter` that sets `IS_STICKY_ATTRIBUTE = true` on the exchange. Works with the standard `lb://` URI scheme and composes with other filters:

```yaml
spring:
  cloud:
    gateway:
      server:
        webflux:
          global-filter:
            web-session-sticky-load-balancer:
              enabled: true
          routes:
            - id: legacy-route
              uri: lb://legacy-service
              predicates:
                - Path=/legacy/**
              filters:
                - WebSessionSticky
            - id: stateless-route
              uri: lb://legacy-service
              predicates:
                - Path=/api/**

/legacy/** requests are pinned to one instance per client session. /api/** requests use normal round-robin. Same service, different behavior per route.

WebSessionStickyLoadBalancer

A ReactorServiceInstanceLoadBalancer that:

• Reads/writes a serviceId -> instanceId affinity map from the gateway WebSession
• Reuses the pinned instance while it remains registered
• Transparently re-pins via round-robin if the instance disappears
• Falls through to the delegate for non-sticky exchanges or non-exchange contexts (e.g. internal WebClient calls)

WebSessionStickyLoadBalancerFilter (alternative opt-in)

A GlobalFilter that handles routes using the sticky:// URI scheme as an alternative to the GatewayFilter approach. Disabled by default.

Key design decisions

• Affinity stored server-side in WebSession, not in a client cookie
• Per-route opt-in via - WebSessionSticky filter (or sticky:// scheme)
• Disabled by default; zero impact on existing deployments
• Uses 2-arg getInstance(serviceId, ReactorServiceInstanceLoadBalancer.class) so a WebSessionStickyLoadBalancer wired via @LoadBalancerClient is resolved correctly per service
• Transparent re-pinning on instance loss (no client-visible error)

Files changed

File	Purpose
WebSessionStickyGatewayFilterFactory.java	Per-route filter that marks exchange as sticky
WebSessionStickyLoadBalancer.java	Load balancer with session affinity logic
WebSessionStickyLoadBalancerFilter.java	GlobalFilter that performs instance selection and URI rewrite for sticky routes
GatewayReactiveLoadBalancerClientAutoConfiguration.java	Wires both beans (disabled by default)
WebSessionStickyLoadBalancerTests.java	6 unit tests for the load balancer
WebSessionStickyLoadBalancerFilterTests.java	10 unit tests for the global filter
WebSessionStickyGatewayFilterFactoryTests.java	4 unit tests for the factory
global-filters.adoc	Documents the global filter
websessionsticky-factory.adoc	Documents the GatewayFilter factory
nav.adoc	Registers the factory doc page

Testing

• 20 new unit tests, all passing
• All 22 pre-existing ReactiveLoadBalancerClientFilterTests and LoadBalancerServiceInstanceCookieFilterTests pass without modification

Checklist

• ASF license header on all new .java files
• @author tags on new and modified files
• Unit tests for all new classes
• Documentation added
• Disabled by default
• DCO Signed-off-by on commit

Related: #1176, spring-cloud/spring-cloud-commons#764

[spencergibb]

Without having reviewed this at all, I'm not sure a new scheme is the way to configure this

[Gebreyesus]

Without having reviewed this at all, I'm not sure a new scheme is the way to configure this

@spencergibb Agreed.
I've updated the PR to provide a WebSessionSticky GatewayFilterFactory as the primary opt-in mechanism. It uses the standard lb:// scheme and applies sticky load balancing per route through configuration:

routes:
  - id: legacy-route
    uri: lb://legacy-service
    predicates:
      - Path=/legacy/**
    filters:
      - WebSessionSticky 

When this filter is applied to a route, every client session is pinned to a specific backend instance for the duration of that session. Affinity is maintained server-side in the gateway WebSession. No new URI scheme required.