Feat: cast true/false values for additional_settings

[kares]

This allows us to properly set additional AWS settings e.g.

output {
  s3 {
    ...
    additional_settings => {
      force_path_style => true
      ssl_verify_peer => false
    }
  }
}

NOTE: could be extended to cast Float/Integer but for now we're mostly after ssl_verify_peer => false ...

[kaisecheng]

Test locally with s3. LGTM