[FLINK-36938] Provide hooks before and after the watermark processing #300

Job	Run time
Pre-compile Checks / Basic QA	8m 16s
Default (Java 11) / Compile	42m 57s
Default (Java 11) / E2E (group 1)	1h 39m 32s
Default (Java 11) / Test packaging/licensing	33m 53s
Default (Java 11) / E2E (group 2)	2h 3m 2s
Default (Java 11) / Test (module: core)	21m 53s
Default (Java 11) / Test (module: python)	28m 10s
Default (Java 11) / Test (module: table)	35m 50s
Default (Java 11) / Test (module: connect)	20m 25s
Default (Java 11) / Test (module: tests)	36m 48s
Default (Java 11) / Test (module: misc)	28m 2s
	7h 58m 48s

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