Get PowerDNS statistics

[styxit]

Extended #45 to support statistics parameters available in PowerDNS 4.3.

Original Pull Request👇

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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)

Types of changes

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

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