Command line option for custom headers

[ivan-gomes]

configurable-http-proxy supports a wide range of configuration options, but it is not practical for it to cover all use cases. An example of an uncovered use case is the Strict-Transport-Security response header. Instead of adding yet another configuration option specifically for this header I propose we implement a --custom-headers <headers> configuration option that allows for user-specified headers to be added to all proxied requests. Upstream projects like https://github.com/jupyterhub/zero-to-jupyterhub-k8s can expose the option to be passed to configurable-http-proxy, for example through the Helm chart.