Support passing empty and binary HTTP header values

Relates to https://gitlab.com/gitlab-com/ops-sub-department/section-ops-request-for-help/-/issues/451.

This is an extract from Draft: Support passing empty and binary HTTP he... (!1906 - closed) that is easier to review. Follow up for Support passing empty and binary gRPC metadata ... (!1927 - merged).

This works fine against staging kas running current code, without protobuf modifications. I.e. this is a backward and forward compatible change:

➜  ~ kg pods --all-namespaces 
Warning: The agent for Kubernetes (agentk) version cannot be checked for compatibility. Agentk and the agent server for Kubernetes (KAS) might not be compatible. Make sure agentk and KAS have the same version.
NAMESPACE     NAME                                      READY   STATUS    RESTARTS       AGE
kube-system   coredns-6799fbcd5-tmf5c                   1/1     Running   34 (41h ago)   74d
kube-system   local-path-provisioner-6795b5f9d8-fwqmj   1/1     Running   25 (41h ago)   62d
kube-system   metrics-server-54fd9b65b-ss4fs            1/1     Running   36 (41h ago)   74d
ns            temp-pod                                  0/1     Error     0              34d
ns            temp-pod1                                 0/1     Error     0              34d