S3 tile writer

Currently implemented within `DataFrame.foreach { ... }`, but would potentially be useful as a sink, accepting some data structure that includes the target URI, bytes to write, whether to gzip, the content-type, and possibly other metadata.

See also #93

Start by examining the existing S3 tile writer inside DataFrame.foreach and read the related issue #93. Define the sink interface around the target URI, bytes, gzip setting, content type, and other metadata; done requires an agreed design and an implementation that replaces or complements the current approach.

Written by the indexing model from the issue text.