Feature to allow headers with an underscore in mesh gateway

Requesting an option to relax the configuration of mesh gateway.

As shown here: https://github.com/kumahq/kuma/blob/1.8.0/pkg/plugins/runtime/gateway/filter_chain_generator.go#L284 all the mesh gateway is configured to reject all requests containing headers with an underscore.

We understand this is set this way so that the following best practices can be followed: https://www.envoyproxy.io/docs/envoy/latest/configuration/best_practices/edge

However, this particular setting is overly restrictive in our environment and we likely cause failures that are too expensive to fix. In particular this is already causing some of our services to fail and halting our ability to deploy the mesh gateway.

We would like either this setting relaxed or configurable.