Add support for extracting LLDP neighbor information

With commit 45fb57573685713350da05d908ca9ed760b60a86, omicron gained the ability to configure a switch's LLDP daemon at rack install time. This allows the rack to announce itself on the customer's network using a common protocol that their network management software should support.

As our rack is announcing itself on the customer's network, it is also collecting information about servers and switches broadcasting announcements to us. Currently that collected information is only available by running lldpadm from within the switch zone. We need to make the collected information available to customers through the rack-level API and CLI.