Inventory of S3 bucket storage usage by types

[ValentinLeTallec]

Hello,

This should resolve #420.

To express storage size, I went with size_bytes: u64 but tell me if you prefer another unit.

[ValentinLeTallec]

Tests are failing on the api version, but I don't think it should be solved in this PR, so I created #729.

[ValentinLeTallec]

For information, the minimal permission I required to run an inventory were :

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Sid": "CloudScannerWatch",
            "Effect": "Allow",
            "Action": [
                "ec2:DescribeInstances",
                "cloudwatch:GetMetricStatistics",
                "cloudwatch:ListMetrics"
            ],
            "Resource": "*"
        }
    ]
}

I started from no permission and added them one by one from the error messages.

I did not need "cloudwatch:DescribeAlarms" (as mentioned in #402 (comment)) neither before nor after creating an alarm.