feat: add a new function StopWatching() to stop the groutine created by WatchConfig()

[chowyi]

I need to stop the WatchConfig() function in certain scenarios. I found someone with the same need in an open Issue: #1404. I have attempted to implement it.

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[sagikazarmark]

Thanks @chowyi!

Unfortunately, this is a breaking change which is not allowed on the v1 release track at the moment.

I'd be happy to accept a change that introduces a new function implementing the same functionality.

[chowyi]

Thanks for your suggestion @sagikazarmark.
I reverted the previous modification, added a new function StopWatching() implementing the same functionality and the unitTest.