Allow a way to set default options to be used for a whole test suite

[twsouthwick]

There are now a number of options that can be set (and infinite combinations with custom MockHandlers). Some of these I find I need to use in the same way for an entire test suite. Currently, I'm using a custom extension method, but the drawback is it has to be called on every builder, ie:

internal static class MyCustomAutoSubstituteExtensions
{
  public static AutoSubstituteBuilder AddCustomExtensions(this AutoSubstituteBuilder builder) => ...
}

public class Test
{
  [Fact]
  public void SomeTest()
  {
    using var mock = AutoSubstitute.Configure()
      .AddCustomExtensions()
      ...
      .Build();
}

This works, but requires every test to manually call this. I propose an API that can be called early on (potentially via C# 9 module initializers) that then gets applied to all tests. It would also be good to provide a way to 'lock' the default set so nothing else inadvertently changes things.

Here's the proposal for the API:

public class AutoSubstitute
{
  public static DefaultAutoSubstituteBuilder Default { get; }
}

public class DefaultAutoSubstituteBuilder
{
  public DefaultAutoSubstituteBuilder ConfigureDefault(Action<AutoSubstituteBuilder> configure);
  public void Reset();
  public DefaultAutoSubstituteBuilder Lock();
}

Some open questions:

• Should we allow default registrations to exist? This isn't supported directly by Autofac, but we can work something around (ie maintain a list of methods to call to register)
• Some methods on AutoSubstituteBuilder will need to be made virtual (and maybe on the SubstituteForBuilder) to gather the submitted configurations. Can we minimize this so that everything funnels through just a couple of methods that can be virtualized to accomplish this?