improve: add license headers to source files (#2980) #5683

Job	Run time
sample_operators_tests (sample-operators/webpage)	4m 9s
sample_operators_tests (sample-operators/tomcat-operator)	3m 44s
sample_operators_tests (sample-operators/mysql-schema)	3m 39s
sample_operators_tests (sample-operators/leader-election)	4m 24s
	15m 56s

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