feat(pojo): add builder methods to Plan and Plan.Root #1038

Job	Run time
Check editorconfig	4s
Security validation	4s
Lint commits for semantic-release	9s
Build Examples	1m 47s
Build and Test Java	4m 50s
osv-scanner (isthmus)	1m 8s
Dry-run release	16s
osv-scanner (core)	1m 12s
osv-scanner (isthmus-cli)	1m 13s
Build Isthmus Native Image (macOS-latest)	21m 35s
Build Isthmus Native Image (ubuntu-latest)	6m 5s
	38m 23s

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