Rusoto: Support generated presigned S3 URLs

[pantaloons]

See equivalent in:

boto3: http://boto3.readthedocs.io/en/latest/guide/s3.html#generating-presigned-urls
Java: http://docs.aws.amazon.com/AmazonS3/latest/dev/ShareObjectPreSignedURLJavaSDK.html
Go: http://docs.aws.amazon.com/sdk-for-go/v1/developer-guide/s3-example-presigned-urls.html

[thudry]

Here is what I got from boto3 implementation so far.

They have a signing type standard, presign-url, presign-post
When singing a request you send the signing type e.g

• Normal request => standard
• Pre signed url => presign-url
• Pre signed post => presign-post

The signing type is then converted into a signature version e.g.

• presign-url => s3v4-query
• presign-post => s3v4-presign-post

that signing version is then used to define which authentication will be used see https://github.com/boto/botocore/blob/55d64656e635ced908f7ba9f427a7c9f42ce8739/botocore/auth.py#L832

To implement only generate_presigned_url() we only need to add presign-url if we also want to do generate_presigned_post() we will also need presign-url

I am fairly new with rust and rusoto but if I read the code correctly the request are always signed the same way.
Before implementing generate_presigned_url() or generate_presigned_post() I would modify the signature flow to allow multiple auth mechanisms.
I'll try to look at the differences between the auth mechanisms tomorrow.

[tomhoule]

We needed this feature for a project so I made a working, very basic implementation that needs cleaning up. I'll try to submit a pull request soon.

[tomhoule]

There's a PR open, but it's proving hard to implement cleanly.

[matthewkmayer]

I believe this is done in https://github.com/rusoto/rusoto/pull/1018 and https://github.com/rusoto/rusoto/pull/1078 . 👍