Add support for IAM Roles

[panbenson]

Feature Request Summary

Support archiving to s3 via IAM roles

What's the motivation or use case for the change/feature?

It looks like the current s3 implementation hasn't been updated in a while (www/lib/S3.php).

As a result, we are only able to use IAM user credentials to authenticate (via archive_s3_key, archive_s3_secret in settings.ini)

WebPageTest/www/archive.inc

Line 196 in 81f5daa

$s3 = new S3(trim(GetSetting('archive_s3_key')), trim(GetSetting('archive_s3_secret')), false, trim(GetSetting('archive_s3_server')), $urlstyle);

Could we please consider adding the option to use IAM roles for better security?