Rusoto: Allow requests to be constructed with a builder pattern

[matthewkmayer]

https://github.com/rusoto/rusoto/pull/789 gave us an idea on what a builder pattern looks like for requests. This is something that'd be nice to have for ergonomics.

Code like

let upload_request = PutObjectRequest {
  bucket: bucket.to_owned(),
  key: dest_filename.to_owned(),
  body: Some(contents.into()),
.  .Default::default()
};

Could be written something like this:

let mut upload_req_builder = PutObjectRequest::new()
  .bucket(bucket)
  .key(dest_filename)
  .body(contents);

[softprops]

2 cent suggestion. Can the builder interfaces be made flexible I terms of the types they accept. If this is issue is about improving ergonomics, I'd love not to have to call to_owned to_string into everywhere. Rather I'd like interfaces that take generic Into types.

[softprops]

This caught my attention and may be of interest. https://vfoley.xyz/rust-compile-speed-tips/ it maybe useful to consider feature flagging builder impls for the sake of compile times. Aws apis are pretty big. There could be a compile time tax for the convenience not everyone wants.

[matthewkmayer]

Yes, taking generic Into types is what we want and is what the PR implemented. I transcribed it in a hurry and incorrectly.

Feature flags sound good. The project will have to pay the time cost in TravisCI but we don't have to force that onto crate users.