add debounce to Dropdown #892

Job	Run time
Build Dash Package	3m 11s
Detect Relevant Path Changes	4s
Typing Tests	2m 25s
Run Background & Async Callback Tests (Python ${{ matrix.python-version }})	0s
Table Unit/Lint Tests (Python ${{ matrix.python-version }})	0s
Table Server Tests (Group ${{ matrix.test-group }})	0s
	5m 40s

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