Would be convenient if this supported Extensions

[j-baker]

See https://blog.adamchalmers.com/what-are-extensions/ for details - much better job of describing than I could.

This is nice, and it'd be really nice if it enabled e.g. adding session management as layers and similar. This is a bit challenging at the moment. Extensions are a huge boon for most webserver-esque frameworks.

[iainmcgin]

Extensions are now supported in buffa (see anthropics/buffa#3), but I need to push a new crate version for that. I'm also working through a stack of PRs to add support for reflection / DynamicMessage which should help with various extension-based interceptors, etc.