Refactor sagemaker-custom-kernel module to use Pydantic

[LeonLuttenberger]

Describe your changes

• Refactor sagemaker-custom-kernel module to use Pydantic

Checklist before requesting a review

• I updated CHANGELOG.MD with a description of my changes
• If the change was to a module, I ran the code validation script (scripts/validate.sh)
• If the change was to a module, I have added thorough tests
• If the change was to a module, I have added/updated the module's README.md
• If a module was added, I added a reference to the module to the repository's README.md
• I verified that my code deploys successfully using seedfarmer apply

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

[malachi-constant]

AWS CodeBuild CI Report

• CodeBuild project: AIOpsModuleTestsInfrastruct-doMLXEYsnmxr
• Commit ID: eac73d4
• Result: SUCCEEDED
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[kukushking]

Thanks!