Maud + Axum extractor/renderer

[emschwartz]

I'm using 3/4 of this stack (haven't needed HTMX yet but I probably will add it at some point soon) and love it. Great idea to coin it as a stack!

It might be nice to include an example of a "layout extractor", which is a struct that is both an axum extractor (to extract common things like whether the user is logged in, or pagination information) and provides a maud layout. David Pedersen, the author of Axum, wrote up the idea in answer to someone's question on Reddit and I think it's a really nice example of a synergy between axum and maud.