This looks really great. I like that everything is defined in plaintext, but is there a way to link in other files? It might help clear up some clutter for large payloads. Environment variables might be good for using secrets more securely as well.

I'm actually working on a similar project (also in Rust) but targeting interactive use and differing environments. It's a tool I created for myself because I was tired of hand crafting curl commands and forgetting which env variables are set to what.

It is still very young but I'll link it anyway: https://github.com/mcastorina/repost