Respect the `ShapeVisitor` methods - `operation_shape.rs` (#2486)

* Move ServerOperationGenerator to ShapeVisitor::operationShape

* Switch operation_shape docs to ModuleDocProvider

* Update codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/ServerRustModule.kt

Co-authored-by: david-perez <d@vidp.dev>

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Co-authored-by: david-perez <d@vidp.dev>