Fix bugs in Event Stream unmarshalling (#736)

* Fix bugs in Event Stream unmarshalling

- S3 Select's `Cont` event doesn't come across with a `:content-type`
  header, but the unmarshaller was expecting this header.
- The unmarshaller attempted to parse events that were modeled as empty
  structs, but S3 sends an empty payload for empty structs rather than
  an empty XML payload conforming to restXml.
- The event stream `Receiver` was returning that there were no more events
  even when it had some remaining events in its buffer.

* Update changelogs

* Incorporate feedback

* Split `smithy_http::event_stream` into multiple modules

* Add another test case to Receiver

* Fix CI

Co-authored-by: Russell Cohen <rcoh@amazon.com>