DeferredSignerSender must be in a stable crate #3370
Description
Currently, DeferredSignerSender is in aws-smithy-eventstream, which is an unstable crate. This type gets added to the ConfigBag, so it must be in a stable crate to avoid potential semver compatibility issues. For example, if we release a new minor version of aws-smithy-eventstream and refer to it in a patch version of aws-runtime, and the generated code in the last SDK release placed it into or pulled it out of a config bag, then event streams would break for these previous SDK versions. This is just generally a subtle hazard that can cause severe issues, as it did for the endpoint prefix (fixed in #3318).
One solution to this would be to make the aws-smithy-eventstream crate a stable crate, which will require some trickery to get working without breaking the current unstable version of aws-smithy-eventstream. The unstable aws-smithy-eventstream will need to depend on its stable counterpart and re-export the DeferredSignerSender from there.