Add codegen helper for defining operation-specific `CustomizableOperation` methods. (#3918)

This PR lays the groundwork for defining operation-specific
`CustomizableOperation` methods.

## Codegen Example

If we were to define pre-signable ops this way, code like the following
would be emitted for each op that supported presigning.

```rust
impl<E, B> CustomizableOperation<crate::operation::put_object::PutObject, E, B> {
    /// Sends the request and returns the response.
    #[allow(unused_mut)]
    pub async fn presigned(
        mut self,
        presigning_config: crate::presigning::PresigningConfig,
    ) -> ::std::result::Result<crate::presigning::PresignedRequest, crate::error::SdkError<E>>
    where
        E: std::error::Error + ::std::marker::Send + ::std::marker::Sync + 'static,
        B: crate::client::customize::internal::CustomizablePresigned<E>,
    {
        self.execute(move |sender, conf| sender.presign(conf, presigning_config)).await
    }
}
```

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