Add `hub.customScopes` directly mapping to `c.JupyterHub.custom_scopes` #2098

Job	Run time
lint_and_validate_rendered_templates	14s
lint_and_validate_templates_with_schema	7s
lint_shell_scripts	43s
lint_and_validate_templates_with_schema (v3.5.0)	13s
test (latest, install)	2m 34s
test (stable, install, --set hub.image.name=quay.io/jupyterhub/k8s-hub-slim --set prePuller.hook....	2m 58s
test (v1.26, install, --set hub.existingSecret=test-hub-existing-secret --set proxy.secretToken=a...	2m 51s
test (v1.25, upgrade, stable, --set proxy.secretToken=aaaa1111 --set hub.cookieSecret=bbbb2222 --...	3m 36s
test (v1.24, upgrade, dev, --set proxy.secretToken=aaaa1111 --set hub.db.type=sqlite-pvc --set si...	5m 3s
test (v1.23, upgrade, 2.0.0, --set proxy.secretToken=aaaa1111 --set hub.cookieSecret=bbbb2222 --s...	4m 3s
	22m 22s

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