[Server] Set http extensions for Request / Response (#963)

...smithy/rust/codegen/server/smithy/ServerCodegenVisitor.kt

...on/smithy/rust/codegen/server/smithy/ServerRuntimeType.kt

.../server/smithy/generators/ServerCombinedErrorGenerator.kt

...rver/smithy/generators/ServerOperationHandlerGenerator.kt

...ver/smithy/generators/ServerOperationRegistryGenerator.kt

...smithy/generators/protocol/ServerProtocolTestGenerator.kt

...en/server/smithy/protocols/ServerHttpProtocolGenerator.kt

...ithy/rust/codegen/smithy/generators/StructureGenerator.kt

...hy/rust/codegen/smithy/generators/error/ErrorGenerator.kt

rust-runtime/aws-smithy-http-server/src/extension.rs

rust-runtime/aws-smithy-http-server/src/lib.rs

rust-runtime/aws-smithy-http-server/src/macros.rs

rust-runtime/aws-smithy-http-server/src/rejection.rs