Add properties for controlling JReleaser configurations

[haydenbaker]

Background

• We have two JReleaser configurations: one for main smithy releases, and one for releasing the CLI itself
• Currently, when running a release, both configurations are used and therefore both releases happen concurrently
• When something goes wrong, it can be difficult to track down the issue, since the logs for each release become intertwined
• By adding a property to control which configuration to apply, we can easily switch between main releases and CLI releases
• We can also remove the stale configuration from the old way of releasing using nexus-staging
• Also, update to latest version of JReleaser (1.12.0)

Testing

• To verify configurations, you can run with --dry-run to see the task order:
  • Do not set a property
  • JReleaser is not configured
  • Set main: ./gradlew jreleaserConfig -P release.main --dry-run
  • JReleaser is only configured for a main release
  • Set cli: ./gradlew jreleaserConfig -P release.cli --dry-run
  • JReleaser is only configured for a CLI release
  • Set both: ./gradlew jreleaserConfig -P release.main -P release.cli --dry-run
  • JReleaser is configured for both (if that's what you really want)

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