[Feature] UserExtensionStorageHelper should support updating multiple values at once.

## Describe the problem this feature would solve

The `UserExtensionStorageHelper` does not offer a way to update multiple values at once, and must be set one at a time. This is a particular inconvenience in the case of key deletion...

### Key deletion issue

The Graph doesn't currently enable a way to delete a single key from a user extension storage container. Instead, the entire storage container must be deleted, recreated, and re-hydrated with values. And because we can't add more than one key at a time, the time/effort/number-of-requests scales with the size of the storage container. This is super inefficient.

## Describe the solution

[Graph supports adding multiple values at once](https://docs.microsoft.com/en-us/graph/extensibility-open-users#1-add-roaming-profile-information), so we should add this functionality to enable the `UserExtensionStorageHelper` to be more efficient.

## Describe alternatives you've considered

None

## Additional context & Screenshots

This is a good opportunity to give feedback to the Graph team on the key deletion issue.

- [ ] Submit issue to Graph SDK for .NET