Added specs and tests for /_plugins/_transform. #1787

Job	Run time
test-opensearch-spec (version=1.3.17, hub=opensearchproject, tests=default)	1m 21s
test-opensearch-spec (version=2.0.0, hub=opensearchproject, tests=default)	1m 20s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=default)	1m 49s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/index_state_management)	53s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/ml)	2m 9s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=routing)	1m 28s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=snapshot)	56s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=dangling)	1m 1s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/replication)	1m 6s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/streaming)	57s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/notifications)	58s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/query_insights)	50s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/workload-management)	1m 5s
test-opensearch-spec (version=2.18.0, hub=opensearchproject, tests=plugins/analysis)	1m 18s
test-opensearch-spec (version=2.19.0, hub=opensearchstaging, tests=default)	1m 44s
test-opensearch-spec (version=3.0.0, hub=opensearchstaging, tests=default)	1m 42s
merge-coverage	10s
	20m 47s

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