Use static method for value reading instead of extension method

This is a rather big change. It changes how the configuration value for a feature switch is being accessed from a custom switch implementation. Instead of typing `this.ReadSwitchFromConfig` and accessing the value via an _extension method_, now the user is advised to access the value using `FeatureSwitch.FromConfig` static method.

The reason for this is a great comment from @amacal, who pointed out that an _extension method_ actually should do anything (reasonable) with the type it is extending. Since the current implementation of the extension method does not do anything with the extended type, it is sort of counter-intuitive to use the extension way at all. Instead, one might expect a separate value access implementation.