Improve flexibility of handlers

[hlbarber]

Motivation and Context

Currently handlers are constrained to OperationShape::Input to Result<OperationShape::Output, OperationShape::Error>. This is overly prescriptive. As long as handler accepts, and returns types which serialize in compliance with the input, output, and error in the Smithy model then we are in compliance with the specification.

This enables:

• The dual of extractors, co-extractors, which insert data local to the handler into the http::Response.
• Differing input/output representations for differing situations.

The change to FromRequest<Ser, Op, B> and IntoResponse<Ser, Op> provides adherence to #2407, paving the way for reusable types.

Description

• Remove OperationShape::Input, OperationShape::Output, OperationShape::Error.
• Change FromRequest<Protocol, B> to FromRequest<Ser, Op, B>.
• Change IntoResponse<Protocol> to IntoResponse<Ser, Op>.
• Accept handlers with arguments FromRequest<Ser, Op, B> and return types IntoResponse<Ser, Op>.
• Introduce struct FixInput and trait FixedService as tools to remove the degree of freedom on UpgradePlugin.

[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)
• Server Test Typescript (ignoring whitespace)

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[github-actions]

A new generated diff is ready to view.

• No codegen difference in the AWS SDK
• No codegen difference in the Client Test
• Server Test (ignoring whitespace)
• Server Test Python (ignoring whitespace)
• Server Test Typescript (ignoring whitespace)

A new doc preview is ready to view.