Added interfaces for Connector and Powerdns

[frankvanhest]

This will PR will add interface for Connector and PowerDNS.

Description

The ConnectorInterface will improve implementing your own connector. In the current version you can only extend the class Connector and make modifications, but you're still limited to the use of Guzzle.

The PowerdnsInterface makes it easier for applications which uses this package to deal with testing when communication with PowerDNS without having to run PowerDNS when running tests.

Motivation and context

This fixes #71

When using dependency injection within your own application it is useful to have interfaces as typehints for function arguments. This way the service container can injection the needed implementation for each situation. When testing your own application you can inject a dummy Powerdns class which implements the interface, so you don't have to run PowerDNS locally. This is extremely useful for behaviour tests which only deals with an applications behaviour and not with infrastructure.

How has this been tested?

n/a

Screenshots (if appropriate)

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

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[trizz]

Hi @frankvanhest,

Thanks for your PR! The changes are looking fine by me, I see no potential problems with this approach. I want to test some other stuff this week, and then I'll merge your PR.