Improve timeout config ergonomics and add SDK default timeouts (#1740)

* Overhaul timeout configuration
* Improve resiliency re-exports
* Refactor Smithy client retry config handling in builder
* Remove non-standard timeout env vars and profile keys
* Break operation customization codegen into its own file
* Move operation customization into a submodule
* Improve operation customization re-exports
* Add integration tests for read and connect timeouts
* Fix connector timeout config in the Smithy `Client` builder
* Rename `HttpSettings` to `ConnectorSettings`