Add OpenAPI spec for Dashboard Secure Embed endpoints #17500

Job	Run time
test / test (3.2, ubuntu-latest)	2m 19s
test / test (2.7, ubuntu-latest)	2m 55s
pre-commit / pre-commit	1m 8s
test / test (jruby-9.4.12.0, ubuntu-latest)	3m 14s
examples / examples	2m 32s
report	7s
	12m 15s

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