[pingdom] Add Pingdom Plugin

[ricoberger]

Is your feature request related to a problem? Please describe

To be the one place for developers it would be nice if kobs can support for Pingdom. That way users can add their defined Pingdom checks to an Application CR, to view the external monitored uptime / response time within kobs.

Describe the solution you'd like

The Pingdom plugin should support the following features in it's first version:

• A list of all defined Pingdom checks, where it should be possible to filter the checks by tag / name.
• The details for a single check.
• All the important metrics measured by Pingdom like response time, uptime and downtime.

Additional context

The API documentation for Pingdom can be found at https://docs.pingdom.com/api/.