Component: HTTP Handlers

[StachuDotNet]

This Issue exists to collect many items that relate to "Http Handlers" in Darklang

In Darklang-classic, HTTP Handlers only existed in our Cloud runtime (since that was the only runtime), but going forward we should have them supported for the Local/CLI runtime as well.

• bring them back locally, in some minimal capacity
• how do custom domains work in this new world? (see feat: Custom Domains UI #3327)
• reimplement LibMiddleware (see Re-implement LibMiddleware #4151 and Create Http middleware functions in stdlib package #3892. also Create a way of adding types to the http framework #3407)
• revisit how we handle responses, esp errors (Surprising (Incorrect?) Behavior with Result Errors returned by an HTTP Handler #4572)
• warm up BwdServer when it starts (see Warm Up BwdServer when it starts #3782)
• look at opts.Limits.MaxConcurrentconnections (likely applies to bwdserver)
• support HTTP Streaming