Introduce s3 client manager and s3 uploaders

[jfzunigac]

This PR enhances the S3writer in a few ways:

1. The uploading mechanism is now pluggable, now uploaders have to extend S3Uploader class. This change simplifies the implementation of uploads to S3 using alternative APIs, such as MultiPartUpload or TransferManager. This ensures backwards compatibility so newer implementations are easier to roll out. New config is added to reference the uploader class: optional string uploaderClass = "com.pinterest.singer.writer.s3.PutObjectUploader"
2. Add support for uploading to buckets in different regions. The region needs to be specified when creating a new S3Client, this means one Singer instance can't upload to multiple buckets in different regions. To enable this, we introduce a new S3ClientManager which will create a new client per region. New S3Writer config for region: optional string region = "us-east-1". Additionally, we remove the S3Client.close() call in S3Writer since clients are shared resources.
3. Use S3ObjectUpload to encapsulate an upload. Useful if metadata needs to be attached to each upload in the future
4. Rename ObjectUploaderTask -> PutObjectUpolader to match pluggable uploader class. Functionality remains the same