refactor(axum): porting API from `rocket` to `axum`

[LukeBrandon]

Description of changes:

• This solves: #30
• Basic axum web server running with connection to DB
  • Followed axum sqlx postgres example and this article
• Updated relevant README.md's
• Created fluke::configuration module for loading environment variables
• Messages model
  • [x] GET /messages
  • [x] GET /messages/:id
  • [x] POST /messages
  • [x] PUT /messages/:id
  • [x] DELETE /messages/:id
• Users model
  • [x] GET /users
  • [x] GET /users/:id
  • [x] POST /users
  • [x] PUT /users/:id
  • [x] DELETE /users/:id

The goal is to get the axum API up to the point of equivalence with the rocket implementation.