Add MethodInterceptor extension point with feign-validation and feign-http-cache modules

[velo]

Summary

• New feign.MethodInterceptor extension point: an around-style interceptor that wraps the entire HTTP exchange after Contract resolution. It has access to raw method arguments, MethodMetadata, the resolved RequestTemplate, and (after chain.next() completes) the captured Response. Marked @Experimental.
• New feign-validation and feign-validation-jakarta modules: BeanValidationMethodInterceptor runs JSR-303 / Jakarta Bean Validation against the body argument and any @Valid-annotated parameters before the request is dispatched. Constraint violations surface as ConstraintViolationException.
• New feign-http-cache module: HttpCacheInterceptor adds conditional If-None-Match / If-Modified-Since headers, stores 2xx responses with ETag / Last-Modified validators, and short-circuits to the cached value on 304 Not Modified. Pluggable HttpCacheStore SPI plus an InMemoryHttpCacheStore default.

Why

RequestInterceptor only sees the encoded RequestTemplate; ResponseInterceptor only sees the response. Use cases like body validation (needs the typed object before encoding) and HTTP cache (needs both the request and the response, and may short-circuit) needed a third extension point. The two new modules ship as flagship implementations.

Test plan

• mvn -Pdev clean install -DskipITs (full reactor, 54 modules, all green)
• core/src/test/java/feign/MethodInterceptorTest.java covers chain order, short-circuit, exception propagation, and Invocation.response() visibility.
• Validation modules tested for: valid body passes, invalid body throws before any request is sent, @Valid on non-body parameters honoured, no-arg methods pass through.
• HTTP cache tested for: ETag / Last-Modified storage, 304 short-circuit, fresh 200 replaces cached entry, non-validators not stored, POST bypasses, Cache-Control: no-store skipped, 5xx propagates without evicting.