[Router] Add composable scoring and eligibility policies (#37731)

Co-authored-by: inkcherry <mingzhi.liu@amd.com>

experimental/sgl-router/tests/proxy/common/cache_aware_fixture.rs

@@ -29,6 +29,8 @@ pub fn config() -> Config {
             circuit_breaker: None,
             cache_aware: Some(CacheAwareConfig::default()),
             sticky: None,
+            fused: None,
+            eligibility: None,
         },
         discovery: DiscoveryBackend::StaticUrls(StaticUrlsDiscoveryConfig {
             urls: vec!["http://placeholder:0".into()],