Event stream protocol implementation

[rcoh]

• Event Stream frame encoding/decoding (Implement Event Stream message frame ser/de in smithy-eventstream #609)
• http::Response -> operation::Response refactoring in smithy-http (Refactor middleware to use new operation::Response instead of http::Response<SdkBody> #635)
• Refactor sigv4 to sign arbitrary data rather than http::Request
• Track the initial signature in the property bag
• Rust implementation for Event Stream sender in input
• Rust implementation for Event Stream receiver in output
• Codegen
  • Event Stream input sender w/ signing
  • Event Stream output receiver
  • Event Stream traits
  • Selection of HTTP protocol version via Smithy traits (e.g., @restXml)
    • Current service models aren't using these correctly, and some operations actually require HTTP 1 for bidirectional event streaming
    • For now: error out if there's an operation that doesn't specify HTTP 2 for bidirectional streaming, and customize models for known bad operations to specify HTTP 2
  • Events marked with @error are terminal; should filter them out of the event stream union and handle them as actual SdkErrors
  • Make Event Stream recv() errors compatible with other SdkErrors when using the ? operator
  • Non-HTTP bound protocols like AWS JSON send initial members as a struct embedded in a first event frame
  • Generate new error type for Event Stream recv() since it's not 1-to-1 with the operation errors
• Testing
  • Record bidirectional traffic against all supported protocols:
    • restJson1
    • awsJson1 and awsJson1_1
    • awsQuery
    • ec2Query
    • restXml
  • Exercise all event stream header types
  • Exercise all event stream payload types (blob, string, struct, union)
  • Write replay tests
• Test the developer experience when the Event Stream input stream yields an error (where does the error go?)