[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

Start by locating UserExtensionStorageHelper and review the linked Microsoft Graph documentation for adding multiple values at once. Confirm how the helper currently updates individual values, then define completion as supporting one operation for multiple updates while addressing the storage-container key deletion workflow.

索引モデルが issue の本文から書いたものです。