Add the Definition and Implementation of IFWXDataFilter Methods

Now add the code to define and implement the IFWXDataFilter methods.

To define and implement the IFWXDataFilter methods

1. Add the definition of IFWXDataFilter methods:

   Copy the prototypes of the methods of IFWXDataFilter from wspfwext.h and add them to smtpdatafilter.h.

   Add the declaration of the methods to smtpdatafilter.h.

2. Add the implementation of IFWXDataFilter Methods

   Use wspfwext.h to get the prototypes of the methods of IFWXDataFilter and add them to SMTPDataFilter.cpp with implementation code.

   Start with a simple implementation in SMTPDatafilter.cpp; for example,

   return E_NOTIMPL;