Investigate whether we would want to deprecate `HTTPRequest` in favor of `swift-http-types`

Apple [just announced](https://www.swift.org/blog/introducing-swift-http-types) a new package containing client- and server-independent HTTP types, which we should probably recommend to downstream consumers. Conveniences such as fetching JSON, HTML or XML documents could then be built on top of their `HTTPRequest` (which looks quite similar to our implementation too).

Start by comparing this library's HTTPRequest implementation with the HTTP types in swift-http-types, then review the package's current downstream-facing usage. Determine whether deprecation is appropriate and document the recommendation for consumers, including how JSON, HTML, or XML conveniences would relate to the replacement.

Written by the indexing model from the issue text.