Add collector for PCIe devices with link information

[naoki9911]

The link status of PCIe devices sometimes changes, like link or speed downgrades, and devices disappear. This patch collects PCIe devices' link infromation to detect such failures.

As a first step, this collector exports PCIe devices'

• Device information (vendor_id, device_id, etc.)
• Parent PCIe device (e.g. PCIe bridge, PCIe switch)
• Link status (max_link_{speed|width}, current_link_{speed|width})

This depends on https://github.com/prometheus/procfs/pull/728 https://groups.google.com/g/prometheus-developers/c/0GJTs2OjvCs