feat: Publish releases on Swift Package Registry

[jbelkins]

Issue #

#1553

Description of changes

• Reorganize directories so that the SDK can be developed in either form: Registry-based Swift packages or the current, Git-hosted Swift package.
• Set cache headers on Registry files to help speed development.
• For first round of Registry testing, smithy-swift will remain hosted on Git. Codegen and runtime packages have been updated to reflect.
• Add a parameter to the Gradle stageSdks task that controls whether to copy package manifests over for the service clients, i.e.:

./gradlew -p codegen/sdk-codegen stageSdks -PwithManifests

• Some code cleanup.

New/existing dependencies impact assessment, if applicable

No new dependencies were added to this change.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.