Calling public HTTP initializer with URL without a scheme results in a precondition crash

We called the [`HTTPRequest` initializer `.init(method: Method = .get, url: URL, headerFields: HTTPFields = [:])`](https://github.com/apple/swift-http-types/blob/main/Sources/HTTPTypes/HTTPRequest%2BURL.swift#L68) mistakenly with a url without a scheme.

This resulted in a crash in our application due to a [precondition on the url parameter to have a scheme](https://github.com/apple/swift-http-types/blob/e9b43cc452707206dd5275ab85c4ea24345d19f0/Sources/HTTPTypes/HTTPRequest%2BURL.swift#L133)

This seems like it really isn't public API if there are input constraints that the complier can't check for but result in a runtime crash if violated. Should there be a similar, failable api that gracefully handles type-valid input that fails internal conditions?

Start in Sources/HTTPTypes/HTTPRequest+URL.swift at the public URL initializer around lines 68 and 133. Read how the URL scheme precondition is enforced and review the issue's proposed failable-API direction. Done means the project has an agreed behavior for scheme-less URLs and corresponding coverage for the public initializer.

Written by the indexing model from the issue text.