Support extra pod spec for user placeholder pods #2696

Job	Run time
lint_shell_scripts	1m 23s
lint_and_validate_rendered_templates	9s
lint_and_validate_templates_with_schema	12s
test (latest, install)	1m 54s
lint_and_validate_templates_with_schema (v3.5.0)	15s
test (stable, install, --set hub.image.name=quay.io/jupyterhub/k8s-hub-slim --set prePuller.hook....	1m 57s
test (v1.31, install, --set hub.existingSecret=test-hub-existing-secret --set proxy.secretToken=a...	1m 36s
test (v1.30, upgrade, stable, --set proxy.secretToken=aaaa1111 --set hub.cookieSecret=bbbb2222 --...	2m 55s
test (v1.29, upgrade, dev, --set proxy.secretToken=aaaa1111 --set hub.db.type=sqlite-pvc --set si...	3m 1s
test (v1.28, upgrade, 2.0.0, --set proxy.secretToken=aaaa1111 --set hub.cookieSecret=bbbb2222 --s...	3m 20s
	16m 42s

Provide feedback