Resolve architects' review feedback for Key Vault

[heaths]

Feedback from our architects:

• Delete methods on the KeyVaultAccessControlClient should return None and ignore HTTP 404s.
• KeyVaultRestoreOperation and KeyVaultSelectiveKeyRestoreOperation can be removed
• KeyVaultBackupClient.begin_restore and .begin_selective_key_restore can be merged into a single method -- a key_name kwarg will determine the behavior
• KeyVaultBackupOperation should only have a folder_url property
• Restore operations should return LROPoller[None], and backup operations should return LROPoller[KeyVaultBackupOperation]
• Instead of get_backup_status and get_restore_status, pollers should be rehydrate-able with a simplified continuation token (e.g. the value of the response's azure-asyncoperation header instead of a pickled PipelineResponse)
• role_* parameter names should be shortened as much as possible without ambiguity

Feel free to update this issue with idiomatic feedback.