Add presigned request support for Polly (#735)

* Split out MakeOperationGenerator for HttpProtocolGenerator

* Clean up body metadata calculation

* Add debug instructions to readme

* Add example for Polly presigned requests

* Add ability to change payload signing type for query signing

* Generate special cased make operation functions for presigning

* Reorganize protocol generators

* Rename ProtocolConfig to CodegenContext and move to a central location

* Consolidate protocol classes in protocols package

* Make protocol generator names consistent

* Fully decompose ProtocolGenerator

* Add integration test for Polly presigning

* Add make_operation and presigned to reserved words

* Update presigning RFC

* Make Polly transforms reusable and add tests

* Fix RequestBindingGeneratorTest

* Update changelogs

* Remove `HttpBodySigningType`

* Perform model transformations up front

* Add checks to presigning transformers

* Fix `codegen-server` build