Add support for custom default value generators (i.e. make default value providers part of the public API)

[stakx]

This is a follow-up to #366, where I said:

I'm closing this briefly. I suspect the API could be simplified a little more.

Almost half a year later, I'm back with this PR. It allows implementation of custom default value providers:

sealed class CustomDefaultValueProvider : DefaultValueProvider
{
    protected override object GetDefaultValue(Type type, Mock mock)
    {
        return …; // any instance of the requested `type`
    }

    // optionally override `GetDefaultParameterValue` and/or `GetDefaultReturnValue`
    // if you need to know about the context (i. e. what Moq needs a default value for).
}

Which is massively simpler than what the earlier PR proposed. A mock—or mock repository—can then be set up to use that provider as follows:

mock.DefaultValueProvider = new CustomDefaultValueProvider();

(For backwards compatibility and ease of use, Mock.DefaultValue can still be set to DefaultValue.Empty or DefaultValue.Mock.)

This is meant to be both an extension point for better interoperation with libraries such as AutoFixture, and as a potential prerequisite for resolving #173 and #330 (which ask for functional changes regarding DefaultValue.Empty, which possibly means its behavior should be configurable in order to not introduce breaking changes).