feat: add Shadow plugin to build shaded Kotlin jars

[tjbanghart]

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Adds the Shadow plugin. This will let us build shaded jars that include all dependencies so developers can use the Kotlin SDK as a standalone jar in other JVM projects.

Fixes #1344

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[tjbanghart]

Yeah this works as expected!

Although, I think https://github.com/looker-open-source/sdk-codegen/pull/1343 should get merged first since it includes a similar gradle + kotlin version changes. I snuck a gradle version bump in here.