Some SafeFileApiNativeMethods do not supports OAuth credential

[dudasl]

These function from Microsoft.InformationProtectionAndControl::SafeFileApiNativeMethods supports pass only SymmetricKeyCredential for credential:

• IpcfEncryptFile
• IpcfEncryptFileStream
• IpcfDecryptFile
  When you working against Rights Management Servers that have setup OAuth2 authentication, you need pass your OAuth2CallbackContext instead symmetric credential.

Code looks prepared for OAuth2, because all these functions calls SafeNativeMethods.CreateIpcPromptContext which supports OAuth2.

Note: You can inspire with my PR #25.

[pofman]

@kkanakas is any update / sample code regarding OAuth2 support?