Produce two publishable bundles in CI (#986)

* Move the publisher tool from `aws-sdk-rust`

* Add `rust-runtime:assemble` target to generate a publishable bundle

* Run `fix-manifests` on assemble output

* Produce publish-ready Smithy runtime bundle during CI

* Allow publish from any arbitrary directory

* Add safe-guard to prevent accidental publish from local dev

* Fix unit test target

* Incorporate feedback

* Add `buildSrc` tests and publisher tool to CI