Optionally download tags when getting object

[mdelillo]

This adds a new param to in requests called download_tags. It will write the S3 object tags as key:value pairs to tags.json. It is disabled by default.

The SetTags function on s3client is currently only used for testing, but it could be used to allow tags to be written in out (#119).

[xtremerui]

Hi @mdelillo , it seems there is test failure on both alpine and ubuntu build.

• Failure in Spec Setup (BeforeEach) [0.573 seconds]
in
/go/src/github.com/concourse/s3-resource/integration/in_test.go:21
  when download_tags is true
  /go/src/github.com/concourse/s3-resource/integration/in_test.go:442
    writes the tags to tags.json [BeforeEach]
    /go/src/github.com/concourse/s3-resource/integration/in_test.go:494

    Expected error:
        <s3.requestFailure>: {
            RequestFailure: {
                awsError: {
                    code: "AccessDenied",
                    message: "Access Denied",
                    errs: nil,
                },
                statusCode: 403,
                requestID: "8C772CDF23216B9B",
            },
            hostID: "ftWDFFDn+uvPyhLC4DjQTX2DmU0ErgjfA5MpKqPduWwx+YYjUrUlOINGcHd7UdYtGfyLr7rl9ik=",
        }
        AccessDenied: Access Denied
        	status code: 403, request id: 8C772CDF23216B9B, host id: ftWDFFDn+uvPyhLC4DjQTX2DmU0ErgjfA5MpKqPduWwx+YYjUrUlOINGcHd7UdYtGfyLr7rl9ik=
    not to have occurred

    /go/src/github.com/concourse/s3-resource/integration/in_test.go:486

[mdelillo]

From the error it seems like the S3 token doesn't have permission to write object tags. I actually ended up solving my original problem in a different way, so if this isn't easily mergeable then I'll probably close it out.

[xtremerui]

I have config the IAM to have right permissions for the tests it looks good now. Will try to move this PR forward.

[xtremerui]

@mdelillo all looks good. The only concern is the IAM change that needed for using download_tags. Could you update the readme either at https://github.com/concourse/s3-resource/blob/7b1ba3b876112353df0f78eef8b67ea7e3912fb4/README.md#required-iam-permissions or where you added download_tags to make the usecase clear? Thx!

[mdelillo]

That makes sense, I added it to Required IAM Permissions. I also added a section in the integration tests since the s3:PutObjectTagging is only needed during testing.