Add `BulkDeleteDatastoreItems` to Datastore API spec #15514

Job	Run time
pre-commit / pre-commit	1m 42s
examples / examples	2m 2s
test / test (3.2, ubuntu-latest)	1m 18s
test / test (2.7, ubuntu-latest)	1m 34s
test / test (jruby-9.4.12.0, ubuntu-latest)	3m 7s
report	9s
	9m 52s

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