Authenticate with //blp/apiauth

[ssgonell]

When we attempt to connect with app_name, xbbg connects fine but does not actually do the process of authentication using //blp/apiauth service as mentioned in the documentation (see screenshot from Core Developer Guide, section 3.1).

Rblpapi does it as expected, see: https://github.com/Rblp/Rblpapi/blob/master/src/authenticate.cpp