Add PTP support into assets

[Oleg Rakhmatulin]

There are several places for updates to make PTP support available in .NET SDK:

• Add the new boolean property IsPtpNoExceptionEntry into the IAccountConfiguration interface implementation
• Add the new enum AssetAttributes and related Attributes property into the IAsset interface implementation
• Add new filtering property Attributes into the AssetsRequest class for filtering by PTP attributes