Callback for obtaining all contents for signing, if not provide during creating of signature context.
ASDSoft.WebSigner.Server (in ASDSoft.WebSigner.Server.dll) Version: 2.5.0
This method is called by the infrastructure for example when a signed document requires certificate included in signing content.
Othervise can implement only NotSupportedException
Implementation must be thread safe, when sign more than one content per session.