RFC30/Set fields to fluent builder

[thomas-k-cameron]

Motivation and Context

Part of RFC30

Description

Add fn set_fields to fluent builder.

Testing

NA

Checklist

NA

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[Velfi]

Having a crate-specific or even generic pseudo-code example would be helpful here. Without that, it's hard to understand how this will interact with the existing code. The doc comment says

"This function replaces the parameter with new one."

but it looks like it would replace ALL parameters with whatever was in InputBuilderType.

[thomas-k-cameron]

#{InputBuilderType} is the builder type of the operation.
So, if the fluent builder you are building is S3's aws_sdk_s3::client::fluent_builders::CopyObject then the builder type would be crate::input::copy_object_input::Builder.

generated code would look like this.

    pub struct CopyObject {
        handle: std::sync::Arc<super::Handle>,
        inner: crate::input::copy_object_input::Builder,
    }
    impl CopyObject {
        /// Creates a new `CopyObject`.
        pub(crate) fn new(handle: std::sync::Arc<super::Handle>) -> Self {
            Self {
                handle,
                inner: Default::default(),
            }
        }

         #[cfg(aws_sdk_unstable)]
        /// This function replaces the parameter with new one.
        /// It is useful when you want to replace the existing data with de-serialized data.
        pub fn set_fields(
            mut self,
            data: crate::input::copy_object_input::Builder,
        ) -> Self {
            self.inner = data;
            self
        }

[thomas-k-cameron]

there is a bug. let me fix it up

[thomas-k-cameron]

@Velfi
Would you kindly review this one?