Add Scheme extractor

[bengsparks]

• I have looked for existing issues (including closed) about this

Feature Request

I'd like an easy way to see the scheme / protocol section of the request, which can be found in various headers and the URI itself.

Motivation

Similar to #826, extracting of the scheme of a URI is desirable.

Proposal

use axum::extract::Scheme;

async fn foo(Scheme(scheme): Scheme) -> ...

Alternatives

Manually extracting the scheme via the parts of a URI / analyzing various headers, which is a tedious process.

If this is something that should be part of axum, I can submit a Pull Request with the implementation I already have (adapted from the existing extractor for hosts), and follow it up with some tests.

[davidpdrsn]

Sure! If that’s possible it would be nice to have.