fuzz: route lookup and header finalization fuzzer. (#4116)

This was a pretty easy small test to add to gain coverage over all route resolution and header
finalization. Automatically generated corpus from //test/common/router:config_impl_test. To regenerate the corpus:

bazel run //test/common/router:config_impl_test \
     --test_env="ROUTE_CORPUS_PATH=$PWD/test/common/router/route_corpus"
Risk level: Low
Testing: Corpus coverage of 72.4% for config_impl.cc.

Signed-off-by: Harvey Tuch <htuch@google.com>

test/common/router/BUILD

@@ -14,13 +14,15 @@ envoy_cc_test(
     name = "config_impl_test",
     srcs = ["config_impl_test.cc"],
     deps = [
+        ":route_fuzz_proto",
         "//source/common/config:metadata_lib",
         "//source/common/config:rds_json_lib",
         "//source/common/http:header_map_lib",
         "//source/common/http:headers_lib",
         "//source/common/json:json_loader_lib",
         "//source/common/router:config_lib",
         "//source/extensions/filters/http/common:empty_http_filter_config_lib",
+        "//test/fuzz:utility_lib",
         "//test/mocks/server:server_mocks",
         "//test/test_common:environment_lib",
         "//test/test_common:registry_lib",
@@ -83,6 +85,28 @@ envoy_cc_test(
     ],
 )
 
+envoy_proto_library(
+    name = "route_fuzz_proto",
+    srcs = ["route_fuzz.proto"],
+    generate_python = 0,
+    deps = [
+        "//test/fuzz:common_proto",
+        "@envoy_api//envoy/api/v2:rds_cc_proto",
+    ],
+)
+
+envoy_cc_fuzz_test(
+    name = "route_fuzz_test",
+    srcs = ["route_fuzz_test.cc"],
+    corpus = "route_corpus",
+    deps = [
+        ":route_fuzz_proto",
+        "//source/common/router:config_lib",
+        "//test/fuzz:utility_lib",
+        "//test/mocks/server:server_mocks",
+    ],
+)
+
 envoy_cc_test(
     name = "router_ratelimit_test",
     srcs = ["router_ratelimit_test.cc"],