Allow clear in onyx update

[Beamanator]

cc @marcaaron @Julesssss @Justicea83 since y'all are reviewing the related Web-E PR 👍

Details

As noted by @marcaaron here, we use Onyx.clear() when signing out of NewDot. This is fine but in the recent API refactor we need to call Onyx.clear in a few places after closing an account (the related PRs & issue) & maybe even after signing in

Currently we do Onyx.clear().then because Onyx.set is faster than Onyx.clear. We won't have to use .then if we use Onyx.merge though b/c Onyx.merge is slower than Onyx.clear

• Investigation here: Set default key states when clearing the cache #129 (comment)
• Comments in code here:

  react-native-onyx/lib/Onyx.js

  Lines 721 to 735 in 445769a

  	* Note that calling Onyx.clear() and then Onyx.set() on a key with a default
  	* key state may store an unexpected value in Storage.
  	*
  	* E.g.
  	* Onyx.clear();
  	* Onyx.set(ONYXKEYS.DEFAULT_KEY, 'default');
  	* Storage.getItem(ONYXKEYS.DEFAULT_KEY)
  	* .then((storedValue) => console.log(storedValue));
  	* null is logged instead of the expected 'default'
  	*
  	* Onyx.set() might call Storage.setItem() before Onyx.clear() calls
  	* Storage.setItem(). Use Onyx.merge() instead if possible. Onyx.merge() calls
  	* Onyx.get(key) before calling Storage.setItem() via Onyx.set().
  	* Storage.setItem() from Onyx.clear() will have already finished and the merged
  	* value will be saved to storage after the default value.

Related Issues

https://github.com/Expensify/Expensify/issues/213701

Automated Tests

Not needed, just adding the ability to have clear be an onyxMethod

Note: This PR was tested in combination with:

• [Refactor] Use CloseAccount to replace User_Delete and refactor related pages App#9635
• https://github.com/Expensify/Web-Expensify/pull/34159

Results here: https://github.com/Expensify/Web-Expensify/pull/34159#issuecomment-1179061881

Linked PRs

https://github.com/Expensify/App/pull/9635/files

[github-actions]

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[Beamanator]

I have read the CLA Document and I hereby sign the CLA

[Justicea83]

Code looks pretty straight forward to me