Support using httpbin without flasgger

[mgorny]

Make the dependency on flasgger optional. The dependency has been added relatively recently (i.e. before the original package was abandoned but after its last release), and it is only used to provide a more dynamic landing page. This is unnecessary for use of httpbin for testing, and it introduces an indirect dependency on Rust that is problematic.

With this change, flasgger is no longer installed by default. It can be enabled via "[flasgger]" extra. When flasgger is not available, httpbin redirects to the "legacy" index page.