Add Sankey widget definition to Dashboards api specs #16219

Job	Run time
pre-commit / pre-commit	1m 38s
test / test (jruby-9.4.12.0, ubuntu-latest)	2m 54s
test / test (3.2, ubuntu-latest)	1m 7s
test / test (2.7, ubuntu-latest)	1m 39s
examples / examples	2m 8s
report	7s
	9m 33s

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