[cobalt][c++ client lib] Adds new factory method NewCobaltLoggerForProjectName

Previously this library only supported connecting to Cobalt via the
older, heavier technique of passing a VMO containing the registry bytes.
Now it also supports connecting via the newer, lighter technique of
passing just the project name.

Test:
I added a new test case to cobalt_logger_unittest.cc

Change-Id: Id90ff6fc67faf73d7b19d9dc8a9691adee48de49