Pageant is PuTTY's SSH authentication agent. Multiple other FOSS projects like WinSCP use it too.
There exists a patch for libssh-0.5.3 (through 0.5.5) to support Pageant. However, it does not apply cleanly to 0.6.3.
Currently on the X2Go Project, X2Go Client for Windows is stuck using libssh-0.5.5 because many of our users rely on Pageant support. We have bug 590 written for us to migrate to libssh 0.6.x with the Pageant patch applied.