Move BindingContext and related types to NamingConventionBinder project

[adamsitnik]

In order to remove BindingContext from InvocationContext (a step toward replacing InvocationContext with ParseResult in command handler/action), I've moved it to NamingConventionBinder and made the handler responsible for keeping the reference to BindingContext.

namespace System.CommandLine.NamingConventionBinder
{
    public abstract class BindingHandler : ICommandHandler
    {
        private BindingContext? _bindingContext;

        /// <summary>
        /// The binding context for the current invocation.
        /// </summary>
        public BindingContext GetBindingContext(InvocationContext invocationContext) => _bindingContext ??= new BindingContext(invocationContext);

        public abstract int Invoke(InvocationContext context);

        public abstract Task<int> InvokeAsync(InvocationContext context, CancellationToken cancellationToken = default);
    }
}

Now all the CommandHandler.Create methods return BindingHandler rather than ICommandHandler, and to get the BindingContext the users need to call InvocationContext.GetBindingContext(), which internaly uses ctx.ParseResult.CommandResult.Command.Handler:

public static BindingContext GetBindingContext(this InvocationContext ctx)
{
    return ((BindingHandler)ctx.ParseResult.CommandResult.Command.Handler).GetBindingContext(ctx);
}

Of course it's not that simple, I had to add some workarounds to adjust for the fact that Hosting uses Middleware for dependency injection registration and very late command handler creation.

I know that it ain't pretty, but the only alternative I had was removing NamingConventionBinder project.