[FLINK-28975][connector/base] Add IdlenessManager for main and per-split output in source operator to mark idleness only if both outputs are idle

[PatrickRen]

Unchanged backport of #20760 on release-1.14

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CI report:

• 7bd99c5c9e14d72f947853e70755ac69d3c12835 Azure: FAILURE

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