WebAssembly support

❗ **Are you interested in using the Rust SDK in Web Assembly? Please comment on this issue with more details about your potential use case!** ❗

### Community Note

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**Tell us about your request**

The AWS SDK should be usable from a Rust WebAssembly program.

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

I'm building a web application with core logic written in Rust and compiled to Wasm, and I'd like to connect with AWS APIs such as S3 from my Rust program without having to drop down to JavaScript too often (if ever).

**Are you currently working around this issue?**

Since I'm targeting the web and there's already [an extensive AWS SDK for JavaScript](https://github.com/aws/aws-sdk-js-v3), I can delegate all the AWS bits to JavaScript.

The issue names no files, tests, or entry points. Start by surveying the Rust SDK's compatibility with WebAssembly and its AWS API dependencies; done means the SDK can be used from a Rust WebAssembly program for AWS services such as S3 without routinely dropping to JavaScript.

Written by the indexing model from the issue text.