http_11_proxy: Make inner transport_socket config optional (#36414)

http_11_proxy: Make inner transport_socket config optional

Given that the top-level
[Cluster.transport_socket](1a153166a6/api/envoy/config/cluster/v3/cluster.proto (L1099))
field is optional and defaults to plaintext, this should also be
optional. gRPC is adding support for this transport socket, but they do
not have a `raw_buffer` to explicitly configure. See
https://github.com/grpc/proposal/pull/455#discussion_r1776143739 for
additional context.

Risk Level: Low.
Testing: Existing tests.
Docs Changes: n/a
Release Notes: Done.

---------

Signed-off-by: Tony Allen <txallen@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ c6761de5f33e97758471c6b973a57a5c2e5db925

envoy/extensions/transport_sockets/http_11_proxy/v3/upstream_http_11_connect.proto

@@ -5,7 +5,6 @@ package envoy.extensions.transport_sockets.http_11_proxy.v3;
 import "envoy/config/core/v3/base.proto";
 
 import "udpa/annotations/status.proto";
-import "validate/validate.proto";
 
 option java_package = "io.envoyproxy.envoy.extensions.transport_sockets.http_11_proxy.v3";
 option java_outer_classname = "UpstreamHttp11ConnectProto";
@@ -34,6 +33,6 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // proxy address in ``config::core::v3::Address`` format.
 //
 message Http11ProxyUpstreamTransport {
-  // The underlying transport socket being wrapped.
-  config.core.v3.TransportSocket transport_socket = 1 [(validate.rules).message = {required: true}];
+  // The underlying transport socket being wrapped. Defaults to plaintext (raw_buffer) if unset.
+  config.core.v3.TransportSocket transport_socket = 1;
 }