improve: add license headers to source files #5672

Job	Run time
sample_operators_tests (sample-operators/mysql-schema)	3m 26s
sample_operators_tests (sample-operators/webpage)	4m 4s
sample_operators_tests (sample-operators/leader-election)	4m 17s
sample_operators_tests (sample-operators/tomcat-operator)	3m 46s
	15m 33s

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