Add mechanisms for loading "default HOCON" in both .NET Framework and .NET Core

[Aaronontheweb]

In part to help cleanly decouple HOCON from Akka.NET, one major sticky wicket that needs to be addressed is Akka.NET's usage of the legacy System.ConfigurationManager APIs that are depended on by nearly all of our .NET Framework users (see akkadotnet/akka.net#3944 (comment))

What I would like to implement is a static helper method for loading the "default" HOCON file, if there is one to be found.

The options for the "default" files are, in the following loading order:

1. [.NET Core / .NET Framework] An "app.conf" or an "app.hocon" file in the current working directory of the executable when it loads;
2. [.NET Framework] - the <hocon> ConfigurationSection inside App.config or Web.config, which should also resolve rename AkkaConfigurationSection to HoconConfigurationSection? #8 and Remove top level <akka> element #9
3. [.NET Framework] - and a legacy option, to load the old <akka> HOCON section for backwards compatibility purposes with all users who are running a version of Akka.NET older than 1.4.0.

The default HOCON will be loaded by an ActorSystem automatically in Akka.NET, if no other HOCON value is supplied during the ActorSystem.Create method, via the following call which will be implemented in the HOCON.Configuration library:

ConfigurationFactory.LoadDefaultHocon();

Most of the hard work for this has already been done by @Arkatufus - I'm going to work on implementing a few helper methods and some backwards compatibility shims for making this fully supported inside Akka.NET.

[Aaronontheweb]

Already have this method:

HOCON/src/Hocon.Configuration/ConfigurationFactory.cs

Lines 60 to 63 in 526e6de

	public static Config Load()
	{
	return Load("akka");
	}

I'll use that instead.

[Aaronontheweb]

And look at this, we have this one too:

HOCON/src/Hocon.Configuration/ConfigurationFactory.cs

Lines 86 to 89 in 526e6de

	public static Config Default()
	{
	return FromResource("Default.conf");
	}

[Aaronontheweb]

Alrighty... That's what I'll target.