Ability to replace ConfigurationManager with a custom config provider

[bahusoid]

Fixes #2105

Introduced abstract class ConfigurationProvider exposed as ConfigurationProvider.Current.

To skip all configuration calls (fix for Xamarin) just set it to null:

ConfigurationProvider.Current = null;

Also implemented custom provider for tests under .NET Core (see here)

[hazzik]

not sure what you meant exactly there [#1402] - but it's either already supported by using SystemConfigurationManager or can be easily implemented by end user

There we need a way to implement a configuration from IConfiguration interface introduced in .NET Core 2.0