[DateTimePickerV2] expose onClear callback for single select

[jessieyan]

What package is this for?

• [ x] React
• Angular

Describe the bug

Reported in Graphite https://jsw.ibm.com/browse/GRAPHITE-60929

When user clicks on clear button, they need onClear call back to reset/clear values.

To Reproduce

Steps to reproduce the behavior:

1. Go to single select story
2. Click on calendar button
3. Click on clear button
4. No call back was triggered.

Please create a reduced test case in CodeSandbox:
https://codesandbox.io/s/github/carbon-design-system/carbon-addons-iot-react/tree/next/packages/react/examples/codesandbox

Expected behavior

A clear and concise description of what you expected to happen.

Environment/versions:

• OS: [e.g. MacOS, Windows]
• Browser: [e.g. chrome, safari]
• carbon-addons-iot-react version: [e.g. v2.60.0]

Additional context

If applicable, add screenshots to help explain your problem.

Add any other context about the problem here.

Specific timeline issues / requests

Do you want this work within a specific time period? Is it related to an
upcoming release?

NB: The core contributors will try to work with your timeline, but it's not
guaranteed. The earlier you make a request in advance of a desired delivery
date, the better!