support streaming content from S3, start using it in the webserver

[syphar]

First step of streaming support for content. Adds streaming support to our storage backends (only for testing in the db backend, so no real streaming there). Then uses axum's streaming response feature to stream the responses to the client.

Idea is also to give up size limits since we don't have memory issues any more.

Next steps after this:

• also support rustdoc html content by using the streaming functionality of lol-html. This needs some wrapping since lol-html doesn't do async yet. Probably using the rendering background thread & some channels.
• start with streaming uploads too.

My guess is that all content rendered via askama can't really be streamed, perhaps I switch to a manual streaming impl for bigger streamable things like the sitemap endpoints.

[GuillaumeGomez]

Apart from the nit, looks good to me!

[syphar]

clippy errors are fixed separately in #2850

[GuillaumeGomez]

Much simpler indeed, thanks!