UHV: Move checks for headers with underscores to UHV when it is enabled (#24497)

Move checks for headers with underscores to UHV when it is enabled

While UHV has implemented checks for header names with underscores, the corresponding checks in codec_impl.cc was not disabled. This PR removes this check from codecs when ENVOY_ENABLE_UHV is defined, such that code in UHV is exercised.
Also added missing plumbing for the stats for header map modification done by UHV.

Signed-off-by: Yan Avlasov <yavlasov@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ cdbf0703ac84d0b38edc93018735dd021ad02061

envoy/extensions/filters/network/http_connection_manager/v3/http_connection_manager.proto

@@ -793,7 +793,7 @@ message HttpConnectionManager {
   // normalization for request attributes, such as URI path.
   // If the typed_header_validation_config is present it overrides the following options:
   // ``normalize_path``, ``merge_slashes``, ``path_with_escaped_slashes_action``
-  // ``http_protocol_options.allow_chunked_length``.
+  // ``http_protocol_options.allow_chunked_length``, ``common_http_protocol_options.headers_with_underscores_action``.
   //
   // The default UHV checks the following:
   //