smithy-diff and CLI Git integration

[mtdowling]

It would be awesome if we had a CLI task that integrates smithy-diff with Git so that you could see if the changes that are about to be committed would be a breaking change. We would later add Gradle integration too that wraps the CLI so that you could run something like ./gradlew smithy-diff-check or something.

[mtdowling]

Closing this, but it's something we can consider adding in the future after a CLI is actually released independently.

[mtdowling]

Added in #1724