!str Extend StreamIO API with `idleTimeout` config setting, improve `connectTimeout` model (refs #15401)
!hco Add client-side request rendering and response parsing, refactor, strip-down

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 Remove `Low-Level API Design.rst`
Merge pull request #15403 from spray/release-2.3-dev-http

akka-http-core: remaining functionality for M1