upload/aws: add a sample AWS upload client

This commandline tools uploads a file to S3, as a proof of concept.

All options are mandatory. Credentials are only read from the
commandline and not from the environment or configuration files.

The next step is to add support for importing from S3 to EC2,
currently the images we produce cannot be imported as-is, so this
requires more research.

To try this out: create an S3 bucket, get your credentials and
call the tool, passing any value as `key`. Note that if the key
already exists, it will be overwritten.

Signed-off-by: Tom Gundersen <teg@jklm.no>

go.sum

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