Allow to return statistics endpoint data

[akondas]

Description

Allow to fetch PowerDNS internal statistics:
https://doc.powerdns.com/authoritative/http-api/statistics.html

Motivation and context

I would like to be able to extract statistics with this library.

How has this been tested?

Tested with real PowerDNS instance.

Screenshots (if appropriate)

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[robbinjanssen]

Hi @akondas, thanks for your PR!

Looking good, i'll discuss this within our team tomorrow. We'll also look into why travis is failing and how we can fix this (since that has nothing to do with your PR).

[styxit]

Thanks for contributing! We decided to add support for the available parameter on the statistics endpoint and exclude the ring statistics by default.

The new statistics() method has been released in version v2.5.0

[akondas]

Thanks for such quick release 👍