=str #15355: fix race with ExposedPublisher messaage in tee

 - this change makes the secondary subscription lazy
Merge pull request #15367 from drewhk/wip-15355-tee-first-message-race-drewhk

=str #15355 Fix race in Tee causing a RequestMore overtaking ExposedPublisher
=hco Add ResponseParserSpec, improve message parser
=hco Port over request rendering from spray-can
=hco Make Server header fully configurable (now consistent to User-Agent header on the client side)
!str Extend StreamIO API with `idleTimeout` config setting, improve `connectTimeout` model (refs #15401)
!hco Implement connection-level client API, refactor and strip down client and server infrastructure

Remove everything that is out of scope for akka-http-core M1.
Things like the host- and request-level client-side API will be added back later.
=hco Add ClientServerSpec and TestClient
=hco Remove `Low-Level API Design.rst`
=hco Apply review feedback