Process the request header items. Specifically if present the
Authorization and Expect headers. If the Authorization header is
present, then the callback can validate the user/password. If
authentication fails, the callback should throw an XmlaException
with the correct XmlaConstants values. The XmlaRequestCallback.Helper
class contains the authorizationException method that can be used
by a callback to generate the XmlaException with the correct values.
If the Expect header is set with "100-continue", then it is
upto the callback to create the appropriate response and return false.
In this case, the XmlaServlet stops processing and returns the
response to the client application. To facilitate the generation of
the response, the XmlaRequestCallback.Helper has the method
generatedExpectResponse that can be called by the callback.
Note that it is upto the XMLA client to determine whether or not
there is an Expect header entry (ADOMD.NET seems to like to do this).
The Callback is requested to generate a sequence id string. This
sequence id was requested by the XMLA client and will be used
for all subsequent communications in the Soap Header block.
Implementation can return null if they do not want
to generate a custom session ID, in which case, the default algorithm
to generate session IDs will be used.