AWS example doesn't demonstrate how to do signing on the client

[aduh95]

Initial checklist

• I understand this is a feature request and questions should be posted in the Community Forum
• I searched issues and couldn’t find anything (or linked relevant results below)

Problem

The example only shows how to make signing on the server. Using AWS STS, it's possible to do signing on the client.

Solution

Add an example with signing on the client which uses WebCrypto and a STS session token.

Alternatives

Here are the alternatives I've thought for the actual signing:

• Add the signing algorithm to @uppy/utils, and add a dependency to @uppy/utils in the example package.
• Add the signing algorithm to @uppy/aws-s3-mutlipart.
• Add the signing algorithm as plain JS in the example (not great for readability)
• Add a dependency to the AWS SDK (but the example goal seems to be to show off the integration without any external dependency on the client)

[Murderlon]

How would it look for @uppy/aws-s3-multipart? Just an option like signOnClient: boolean?