traffic agent should be able to preserve http header casing

[tedchang77]

Please describe your use case / problem.
The traffic agent sidecar envoy proxy is converting http headers to lowercase which is breaking our application and downstream services that are case-sensitive.

Describe the solution you'd like
Introduce a traffic-manager flag that will preserve the HTTP header casing in the traffic-agent envoy sidecar. envoy has exposed this configuration in their configs: envoyproxy/envoy#14363

Describe alternatives you've considered
we have upstream services that we are propagating HTTP headers to. these services aren't owned by us, so we can't change them.
we looked at modifying/replacing the envoy config bootstrap-ads.pb in traffic agent sidecar using a configmap volumemount or an init-container, but it looks like this config is generated dynamically when the agent is started.

Versions (please complete the following information)

• Output of telepresence version (just in case the feature exists in future versions)
  Enhanced Client: v2.9.5
  Root Daemon : v2.9.5
  User Daemon : v2.9.5
  Traffic Manager: v2.9.5
• Kubernetes Environment and Version
  GKE Autopilot:
  Client Version: version.Info{Major:"1", Minor:"23", GitVersion:"v1.23.14", GitCommit:"3321ffc07d2f046afdf613796f9032f4460de093", GitTreeState:"clean", BuildDate:"2022-11-09T13:40:19Z", GoVersion:"go1.17.13", Compiler:"gc", Platform:"linux/amd64"}
  Server Version: version.Info{Major:"1", Minor:"24", GitVersion:"v1.24.7-gke.900", GitCommit:"e35c4457f66187eff006dda6d2c0fe12144ef2ec", GitTreeState:"clean", BuildDate:"2022-10-26T09:25:34Z", GoVersion:"go1.18.7b7", Compiler:"gc", Platform:"linux/amd64"}
  Additional context
  Add any other context about the feature request here.
  Without this feature, we cannot use telepresence.

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