Support IAM role for report to S3.

[ohsawa0515]

What did you implement:

Closes #250

How did you implement it:

How can we verify it:

Please create the following IAM role and attach it to the EC2 instance, run vuls report.

{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Effect": "Allow",
            "Action": [
                "s3:ListAllMyBuckets"
            ],
            "Resource": "arn:aws:s3:::*"
        },
        {
            "Effect": "Allow",
            "Action": [
                "s3:PutObject"
            ],
            "Resource": "arn:aws:s3:::vuls/*"
        }
    ]
}

$ vuls report \
    -format-json \
    -to-s3 \
    -aws-region=ap-northeast-1 \
    -aws-s3-bucket=vuls

Todos:

You don't have to satisfy all of the following.

• Write tests
• Write documentation
• Check that there aren't other open pull requests for the same issue/feature
• Format your source code by make fmt
• Pass the test by make test
• Provide verification config / commands
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[kotakanbe]

Many Thanks!