Implement Event Stream message frame ser/de in smithy-eventstream (#609)

* Implement Event Stream message frame ser/de in smithy-eventstream

* Add roundtrip fuzz target to smithy-eventstream

* Add more fuzz testing targets to smithy-eventstream

* Incorporate feedback

* Rename fuzz tests

* Refactor streaming use-case into MessageFrameDecoder

* Add fuzz target for fuzzing the message prelude

* Incorporate feedback