Http Operation traits and structures (#167)

* Http Operation traits and structures

Initial implementation of `Operation` for HTTP based services and `ParseHttpResponse`. I'm also starting to collect design documentation in `design`, an mdBook intended to serve as a repository for public facing design documentation.

* Update rust-runtime/smithy-http/src/body.rs

Co-authored-by: Jonathan Esterhazy <jonathan.esterhazy@gmail.com>

* Add retry policy

* Fix doctest failures

* Update rust-runtime/smithy-http/src/operation.rs

Co-authored-by: Kerem Kat <keremkat@gmail.com>

* Update rust-runtime/smithy-http/src/operation.rs

Co-authored-by: Kerem Kat <keremkat@gmail.com>

* Update design/src/operation.md

Co-authored-by: Kerem Kat <keremkat@gmail.com>

* Update design/src/operation.md

Co-authored-by: Kerem Kat <keremkat@gmail.com>

* Update rust-runtime/smithy-http/src/response.rs

Co-authored-by: Kerem Kat <keremkat@gmail.com>

* Update design/src/operation.md

Co-authored-by: Kerem Kat <keremkat@gmail.com>

* Update design/src/operation.md

Co-authored-by: Kerem Kat <keremkat@gmail.com>

Co-authored-by: Jonathan Esterhazy <jonathan.esterhazy@gmail.com>
Co-authored-by: Kerem Kat <keremkat@gmail.com>