HTTP client protocol specification

Similar to the fetch spec, we'll need not just the Swift shape of a protocol, but also a precise specification that describes the expected behavior of a compliant implementation.

It should go into the expected behavior while streaming, handling cookies, any buffering, and anything else considered "base" behavior (without any add-on feature options).

This would be accompanied by a compliance test suite (#66).

Most of the spec should use "MUST" and "MUST NOT", so that callers can rely on a predictable behavior.

And "SHOULD", "MAY" and similar should come with an explanation of how the caller detects the behavior, and how they should handle both cases (present and absent feature).