Centralize accessing nb metadata #16339

Job	Run time
Lint	2m 21s
cancel_previous_runs	4s
Build VSIX	1m 32s
Type Script Tests (ubuntu-latest, ts-unit)	1m 35s
Type Script Tests (windows-latest, ts-unit)	6m 17s
Tests (raw, python, 3.10, ^[^@]+$\|@mandatory\|@kernelCore\|@python\|@jupyter, ubuntu-latest)	4m 19s
Tests (raw, python, 3.10, @widgets, ubuntu-latest)	4m 29s
Tests (raw, python, 3.10, @iw, ubuntu-latest)	4m 12s
Tests (raw, python, 3.10, @webview\|@export\|@lsp\|@variableViewer, ubuntu-latest)	4m 35s
Tests (raw, python, 3.10, @debugger, ubuntu-latest)	3m 53s
Tests (remote, python, 3.10, ^[^@]+$\|@mandatory, ubuntu-latest)	4m 4s
Tests (remote, python, 3.10, prerelease, ^[^@]+$\|@mandatory, ubuntu-latest)	4m 18s
Tests (web, python, 3.10, ^[^@]+$\|@mandatory, ubuntu-latest)	3m 35s
Tests (raw, python, 3.10, ^[^@]+$\|@mandatory, windows-latest)	6m 23s
Tests (raw, python, 3.10, @widgets, ubuntu-latest, 8)	4m 36s
Tests (remote, python, 3.10, @widgets, ubuntu-latest, 8)	5m 2s
Tests (raw, conda, 3.10, ^[^@]+$\|@mandatory\|@python, ubuntu-latest)	3m 18s
Tests (raw, python, 3.10, prerelease, ^[^@]+$\|@mandatory\|@kernelCore\|@python\|@jupyter, ubuntu-lat...	4m 9s
Tests (raw, noPython, @nonPython, ubuntu-latest)	1m 56s
Tests (remote, python, 3.10, @kernelCore, ubuntu-latest)	4m 23s
Smoke tests (ubuntu-latest, 3.10)	2m 51s
Smoke tests (windows-latest, 3.10)	5m 2s
Smoke tests (macos-latest, 3.10)	4m 19s
Coverage reports upload	0s
	1h 27m 13s

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