Add a new plugin for setting up a trait package

[hpmellema]

Background

Creates a new gradle plugin for quickly defining a smithy trait package. This plugin adds trait codegen to the
smithyBuild dependencies if it does not already exist. The output of the trait codegen plugin is then added to the main java sourceset.

Next steps:

• Add a task to merge service provider files so existing traits can also live in the package.
• Add integration tests
• Possibly have the plugin auto-wire the smithy-model dependency

Testing

Manually tested with a local package.
Additional tests will be added as part of the next steps and will require some additional setup.

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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.