Returned ETag for uploaded entities

[adamsc64]

Summary of Changes

• If 'ETag' is present in service response, return it inside a metadata dictionary for both tasks.CompleteMultipartUploadTask and upload.PutObjectTask.
• This change will allow integrations which rely on these call sites to access the ETag.

Background

• There's a feature request over at boto3 to return ETag information for several call sites, i.e.: S3 upload_file() and upload_fileobj() do not return the object's ETag. boto3#2861.
• In order to achieve this, I believe we need s3transfer to return the ETag information at the call sites changed in this Pull Request.
• Another pull request into boto3 is related, which I have submitted separately here: Returned future result in S3Transfer's download and upload boto3#3316. The two pull requests are completely decoupled, so can be merged in any order or independently.

Design decisions

• I was unsure how to model the entity which would be returning the ETag information. I chose to follow the pattern I found inside upload.UploadPartTask, i.e. a pattern already in the codebase. This pattern returns the ETag inside an outer metadata dictionary.
• Let me know if I should use another strategy, or if this general approach to the problem is problematic for some reason.

[adamsc64]

@kyleknap, I don't know what you think of these changes. Can I get a review? Also open to suggestions if you think someone else is better for the moment. Thanks so much.

[adamsc64]

Hi @kyleknap. New contributor to boto and s3transfer here! Wondering what is the correct way to officially request a review of this by the s3transfer team. Thanks!

[cfxegbert]

It could be nice to return the versionId (optionally None) field also.