Add support for running as a standalone reverse proxy

[sporkmonger]

While Envoy is a good option for integrating into existing infrastructure, it's not a good "Getting Started" experience at all. Given that the project already carries some unavoidable complexity at baseline, the additional complexity of a required Envoy dependency is likely to create excessive friction for new users, even if it does enable some powerful use-cases.

Adding an option to run as a standalone reverse proxy addresses these concerns by giving users a dramatically simpler path to trying the project out. Additionally, it's something of a lightweight & universal compatibility option for environments that don't already have Envoy.