Add support for using activators to resolve TContext

[mgoodfellow]

This PR adds support for more complex use cases (#166) while maintaining backwards compatible support for existing/simple implementations.

New implementations can be added on a per IoC basis. For example the code for SimpleInjector would be:

public class SimpleInjectorCacheContextActivator<TContext> : ICacheContextActivator
	{
		private readonly Container Container;

		public SimpleInjectorCacheContextActivator(Container container)
		{
			Container = container;
		}

		public ICacheContextScope BeginScope()
		{
			return new SimpleInjectorCacheContextScope(Container, AsyncScopedLifestyle.BeginScope(Container));
		}
	}

With the scope similar to:

public class SimpleInjectorCacheContextScope<TContext> : ICacheContextScope
	{
		private readonly Container Container;
                private readonly Scope Scope;

		public SimpleInjectorCacheContextScope(Container container, Scope scope)
		{
			Resolver = resolver;
                        Scope = scope;
		}

		public object Resolve(Type type)
		{
			return Container.GetInstance(type);
		}

		public void Dispose()
		{
                       Scope?.Dispose();
		}
	}

Open to comments and thoughts - this was my initial implementation!

[Turnerj]

Hey @mgoodfellow ! First I want to thank you for submitting the PR - sorry I hadn't got to it myself, between being sick, doing a presentation and making sure my business keeps on spinning I just hadn't got around to it.

I do very much like this change for the reasons you highlighted in your original issue about this. Its kinda late where I am so I'll take another look in the morning but I don't see any obvious issues.

When I merge this, it might be a little bit longer before I release a new version as I need to re-familiarise myself with some serialization changes that are in the main branch. I want to make sure I've got everything done for it that is needed rather than release a half-implemented version. Hopefully that is OK!

[mgoodfellow]

No worries at all! I had been meaning to do this as well and its been on my todo list since we last spoke, always happy to contribute back to projects as we use them internally :) I finally understand background refreshing, so that renewed my interest in adding this functionality.

No rush, speak soon!

[Turnerj]

All looks good - thanks again for the PR!