feat: add Shadow plugin to build shaded Kotlin jars (#1345)

Adds the [Shadow](https://github.com/johnrengelman/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 

## Developer Checklist
[ℹ️](https://github.com/looker-open-source/sdk-codegen/blob/main/CONTRIBUTING.md#developer-checklist)

Thank you for opening a Pull Request! Before submitting your PR, there
are a few things you can do to make sure it goes smoothly:
- [x] Make sure to open an issue as a
[bug/issue](https://github.com/looker-open-source/sdk-codegen/issues/new/choose)
before writing your code! That way we can discuss the change, evaluate
designs, and agree on the general idea
- [x] Ensure the tests and linter pass
- [x] Appropriate docs were updated (if necessary)

kotlin/settings.gradle.kts

@@ -6,5 +6,6 @@ pluginManagement {
         kotlin("jvm") version kotlinVersion
         id("com.diffplug.spotless") version "6.20.0"
         id("org.gradle.toolchains.foojay-resolver-convention") version "0.7.0"
+        id("com.github.johnrengelman.shadow") version "8.1.1"
     }
 }