Fix SDK::Endpoint built-in

[jdisanti]

Setting endpoint_url on a SDK Config currently has no effect due to a bug in deciding if a parameter is a built-in or not in EndpointBuiltinsDecorator. The generated code is placing the endpoint URL into the ConfigBag with the correct aws_types::endpoint_config::EndpointUrl type, but then the operation runtime plugin is trying to pull it out of the ConfigBag with the crate::config::EndpointUrl type, which always yields nothing.

Or, to illustrate with the generated code itself:

    pub fn set_endpoint_url(&mut self, endpoint_url: Option<::std::string::String>) -> &mut Self {
        self.config.store_or_unset(endpoint_url.map(::aws_types::endpoint_config::EndpointUrl));
        self
    }

vs.

let params = crate::config::endpoint::Params::builder()
            .set_endpoint(cfg.load::<crate::config::EndpointUrl>().map(|ty| ty.0.clone()))

This PR adds a unit test that reproduces the issue, and then fixes it.

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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
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[rcoh]

side note: we may want to add a diff target that captures changes like this