Add incomplete Event Stream support with working Amazon Transcribe example (#653)

* Add incomplete Event Stream support with working Amazon Transcribe example

* Make the raw response in SdkError generic

* Fix XmlBindingTraitSerializerGeneratorTest

* Make the build aware of the SMITHYRS_EXPERIMENTAL_EVENTSTREAM switch

* Fix SigV4SigningCustomizationTest

* Update changelog

* Fix build when SMITHYRS_EXPERIMENTAL_EVENTSTREAM is not set

* Add initial unit test for EventStreamUnmarshallerGenerator

* Add event header unmarshalling support

* Don't pull in event stream dependencies by default

* Only add event stream signer to config for services that need it

* Move event stream inlineables into smithy-eventstream

* Fix some clippy lints

* Transform event stream unions

* Fix crash in SigV4SigningDecorator

* Add test for unmarshalling errors

* Incorporate CR feedback