Port Alkane HTTP server to axum

Summary:
The application HTTP API layer is currently built for Rocket 0.4,
which doesn't build anymore on nightly 1.92.

To migrate to Rocket 0.5 seems currently risky, while the HTTP code
doesn't need any of the framework advanced features.

A thin layer on the top of hyper, axum is maintained by the Tokio project.
It's also where former Rocket applications are migrating.

Fixes T2133.

Reviewers: dereckson

Maniphest Tasks: T2133

Differential Revision: https://devcentral.nasqueron.org/D3717