Support for user-agent-prefix setting

With AWS SDK, it is possible to set the property [Configuration#user_agent_prefix](https://docs.aws.amazon.com/AWSRubySDK/latest/AWS/Core/Configuration.html#user_agent_prefix-instance_method) which will be used for every request against AWS services.

Currently, the project I'm working on needs to set this property in order to comply with AWS, they has requested that we identify requests that we make to our users' S3 resources on their behalf so they can assess our impact. The way to identify our requests is by using a custom value for HTTP User-Agent header