Create backend web server implementation with Rust framework

Context

As agreed on in https://github.com/pendulum-chain/tasks/issues/159 and mentioned in #15, we'd like to eventually implement the backend for the pendulum-pay service in Rust. Since the knowledge/experience with Rust webserver frameworks is currently rather limited in the tech team, we want to start exploring this intricacies of implementing and deploying a Rust-based webserver.

TODO

Re-implement the web server with the axum framework. The branch for these changes should base on the offramp-prototype-staging branch.

Remove the contents of the signer-service directory.
In the signer-service directory, start a new cargo project for the Rust web server.
Keep the same functionality as described here intact. The relevant code files for reference are stellar.service.js, stellar.route.js and stellar.controller.js.

Notes

This may or may not replace the current Javascript implementation of the prototype backend. As mentioned in the 'Context' section, the main outcome of this ticket is exploring the respective framework and deployment.

pendulum-chain / vortex