Add Reference Tables API spec #15525

Job	Run time
test / test (3.2, ubuntu-latest)	1m 48s
examples / examples	2m 1s
pre-commit / pre-commit	1m 29s
test / test (jruby-9.4.12.0, ubuntu-latest)	2m 45s
test / test (2.7, ubuntu-latest)	2m 3s
report	6s
	10m 12s

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