Let the user know when a new link connection is made

[crolando]

From @adamsepp 's usecase, they needed to modify plano to learn when links were created. In this task, we will provide a callback that is fired when a new link is created. The callback should be passed enough information to know these facts:

• the donor node
• the recipient node
• Donor node's pin object
• recipient node's pin object
• the link object

See @adamsepp 's commit here:

include/internal/internal.h