SAMLAttributeStatement serializatin and SaveBootstrapContex config

•Extented ToXElement extension methods to support  SAMLAttributeStatment  
•Support for  SaveBootsrapContext  config element in  CreateClaims()  method

I'd prefer this part to be in the `MorePublicSaml2SecurityTokenHandler.CreateClaims()` method instead. It's part of creating the identity so that's where it belongs.