Add support for `libssh2_agent_get/set_identity_path` function

[volks73]

libssh2 v1.9.0 introduced the libssh2_agent_get_identity_path and libssh2_agent_set_identity_path functions. LabVIEW-friendly wrapper functions should be created for each of these functions.

However, as of 2020-03-31, there is no documentation for either of these functions and it is unclear on their functionality. The documentation links at the libssh2 for each functions result in a 404 response. It might be worth holding off on implementing these functions until after some investigation and research has been conducted.