Define OpenAPI spec

Discussion on generating OpenAPI specs for Axum: https://github.com/tokio-rs/axum/issues/50

• Explore aide: https://docs.rs/aide/0.6.0/aide/
• Explore utoipa as alternative to aide
• Explore Poem vs Axum for OpenAPI generation
• Consider linter/tool for suggesting OpenAPI improvements
• Consider feature flag for disabling OpenAPI generation/route. (Not currently interested in maintaining two versions of router code to trim a small amount of bloat. May consider in the future.)
• Consider maintaining OpenAPI spec in git so you don't have to run the app to get the spec
  • Add CI test that verifies this is up to date with the /openapi.json route