Fix event streams in the orchestrator implementation (#2673)

## Motivation and Context
This PR gets event streams working in the client orchestrator
implementation, and depends on #2671.

The orchestrator's `TypeErasedBox` enforces a `Send + Sync` requirement
on inputs and outputs. For the most part, this isn't an issue since
almost all generated inputs/outputs are `Send + Sync`, but it turns out
the `EventStreamSender` wasn't `Sync` due to an omission of the `Sync`
bound. Thus, this PR is a breaking change, as it adds a `Sync`
requirement for anyone who passes a stream to an event stream operation.

----

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Author: jdisanti

CHANGELOG.next.toml

@@ -50,3 +50,15 @@ See the changes in https://github.com/awslabs/smithy-rs/pull/2671 for an example
 references = ["smithy-rs#2671"]
 meta = { "breaking" = true, "tada" = false, "bug" = false, "target" = "client" }
 author = "jdisanti"
+
+[[aws-sdk-rust]]
+message = "For event stream operations such as S3 SelectObjectContent or Transcribe StartStreamTranscription, the `EventStreamSender` in the input now requires the passed in `Stream` impl to implement `Sync`."
+references = ["smithy-rs#2673"]
+meta = { "breaking" = true, "tada" = false, "bug" = false }
+author = "jdisanti"
+
+[[smithy-rs]]
+message = "For event stream operations, the `EventStreamSender` in inputs/outputs now requires the passed in `Stream` impl to implement `Sync`."
+references = ["smithy-rs#2673"]
+meta = { "breaking" = true, "tada" = false, "bug" = false, "target" = "all"}
+author = "jdisanti"

aws/rust-runtime/aws-runtime/Cargo.toml

@@ -7,10 +7,14 @@ edition = "2021"
 license = "Apache-2.0"
 repository = "https://github.com/awslabs/smithy-rs"
 
+[features]
+event-stream = ["dep:aws-smithy-eventstream", "aws-sigv4/sign-eventstream"]
+
 [dependencies]
 aws-credential-types = { path = "../aws-credential-types" }
 aws-http = { path = "../aws-http" }
 aws-sigv4 = { path = "../aws-sigv4" }
+aws-smithy-eventstream = { path = "../../../rust-runtime/aws-smithy-eventstream", optional = true }
 aws-smithy-http = { path = "../../../rust-runtime/aws-smithy-http" }
 aws-smithy-runtime = { path = "../../../rust-runtime/aws-smithy-runtime" }
 aws-smithy-runtime-api = { path = "../../../rust-runtime/aws-smithy-runtime-api" }
@@ -22,6 +26,7 @@ tracing = "0.1"
 uuid = { version = "1", features = ["v4", "fast-rng"] }
 
 [dev-dependencies]
+aws-credential-types = { path = "../aws-credential-types", features = ["test-util"] }
 aws-smithy-protocol-test = { path = "../../../rust-runtime/aws-smithy-protocol-test" }
 proptest = "1"
 serde = { version = "1", features = ["derive"]}