feat(checkpoint-validation): add custom checkpointer validation tool #1712

Job	Run time
check-readmes-synced	2s
Check linting	1m 38s
Check formatting	1m 25s
Build	1m 49s
Validate Deno and Node dependencies in sync	8s
validate-new-notebooks	5s
Unit Tests (ubuntu-latest, 18.x)	1m 37s
Unit Tests (ubuntu-latest, 20.x)	1m 20s
Unit Tests (ubuntu-latest, 22.4.1)	2m 16s
Unit Tests (windows-latest, 22.4.1)	6m 24s
Unit Tests (macos-latest, 22.4.1)	4m 21s
	21m 5s

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