CXX-360 add support for authMechanismProperties

Our SASL implementation seems to only have support for SERVICE_NAME, so I left the others out. They are listed as "MAY IMPLEMENT" in the spec anyway.


I believe that this needs to be a case insensitive comparison (unfortunately):

https://github.com/10gen/specifications/blob/master/source/auth/auth.rst

says

"Naming of mechanism properties MUST be case-insensitive. For instance, SERVICE_NAME and service_name refer to the same property."