Add support for max_receive_message_length in EnvoyExtensionPolicy for ExtProc

*Description*:
I'm seeing `received message larger than max (6133409 vs. 4194304){via_upstream}` when configuring a `ext_proc` and doesn't seem EnvoyExtensionPolicy supports setting `max_receive_message_length`

[optional *Relevant Links*:]
Envoy setting: https://www.envoyproxy.io/docs/envoy/latest/api-v3/config/core/v3/grpc_service.proto#config-core-v3-grpcservice-envoygrpc

Start by reading the Envoy max_receive_message_length documentation linked in the issue and locating EnvoyExtensionPolicy's configuration and translation paths in the repository. Check for existing tests covering ExtProc policy generation; done means the policy can configure this limit and the reported oversized-message failure is addressed.

Written by the indexing model from the issue text.