HttpRequest type upgrade guidance

[rcoh]

HttpRequest when used for mutating HTTP requests sent by the client now refers to aws_smithy_runtime_api::client::http::Request instead of ::http::Request. The new API is intended to be largely drop-in compatible with some caveats:

• Headers are restricted to valid UTF-8 strings. This means that returned values are strings, so conversion into bytes is no longer necessary
• If you need to produce an http = 0.2 HttpRequest, use into_http02x()