Sets the source (USER | OPERATOR | EXTERNAL) of the certificate request fields
specified in the activity which are present in the context. This data is saved
in the serialized 'sources' hash reference in the context and (partly) written
to the database in PersistRequest.
You can also place usernames in source which makes it easy to
identify a person which does something. Yes, this is a kind of a data
If you do not define 'source' then we use the user from the
session to set the source.