[HTTPAPIs] Non-restartable HTTP client body representation

Some HTTP request bodies are naturally non-restartable. There's a discussion about _how to represent that_ on the client body type, and whether HTTP client implementations should be allowed to inspect that information upfront, without having to call the closure again.

One thing to consider in this discussion - bridging from existing AsyncSequences, which mostly aren't restartable, and how to safely bridge them to AsyncReader without crashing on redirects/restarts (since `AsyncSequence.makeAsyncIterator()` is non-throwing, so we can't gracefully throw an error).

Previous discussion on a PR: https://github.com/apple/swift-http-api-proposal/pull/12#discussion_r2585919457