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 log.
If you do not define source then we use the user from the session
to set the source.