add debounce to Dropdown #890

Job	Run time
Detect Relevant Path Changes	5s
Build Dash Package	3m 1s
Typing Tests	2m 45s
Table Server Tests (Group ${{ matrix.test-group }})	-1s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	-1s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	-1s
	5m 48s

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