Move to `axum` and `tokio` stack

[Hirevo]

This PR migrates all of Alexandrie's codebase to use:

• axum instead of tide as its web framework
• tokio instead of async-std as its main async runtime
• tracing instead of slog as the logging mechanism
• tower as the mean to implement middlewares (called layers)

which also implies that we now use:

• http instead of http-types as its HTTP type definitions
• hyper instead of async-h1 as its HTTP parser implementation

This move allows to remove some async runtime duplication that was existing in Alexandrie due to its use of libraries that always assumed to be running a tokio runtime (like rusoto for interactions with S3).

Although I still find tide to be a very nice web framework, its reliance on async-std makes it a bit annoying to work with and can prevent integrations with other library integrations (like middlewares or frontend frameworks like leptos which I am considering using as a replacement for handlebars templating and flash session data especially for pages with rich interactions like the account management page).

The primary goal of this PR is to keep everything working just like before, and therefore is pretty much a 1-to-1 translation from the old to the new framework.
This means that there may be things that may be significantly improved by using more advanced features of the axum framework, like handling the authentication using the Authorization header in the API, which is implemented a bit naively here.
These improvements will come in subsequent PRs.

Closes #159.

[Hirevo]

The migration to Axum is now complete, and the authorization handling has now been improved through the use of custom Auth extractors (one for the frontend based on server-side sessions, and one for the API based on Cargo tokens in Authorization headers).
After the merging of this PR, all other PRs will have to be significantly updated.
I'll try to do this for the ones that I was working on before starting this migration.