Let capabilities supply response interceptors - #3446

Summary

Capability.enrich(ResponseInterceptor) was only called for configured response interceptors. If a capability wanted to supply the response interceptor itself, there was no component for it to enrich, so the interceptor was never installed.

This keeps the existing per-interceptor enrichment path, and adds a default pass-through response interceptor only for the empty-list case. If a capability replaces that default interceptor, the replacement is installed before the response interceptor list is enriched as a whole.

Tests

• ./mvnw -pl core -Dtoolchain.skip=true -Dtest=feign.BaseBuilderTest#capabilityCanProvideResponseInterceptorWhenNoneConfigured test
• ./mvnw -pl core -Dtoolchain.skip=true -Dtest=feign.BaseBuilderTest test
• ./mvnw -pl core -Dtoolchain.skip=true test
• git diff --check

Fixes #2935