Support for execution filters

[Kralizek]

Adding support for execution filters will help minimizing the risk of cross-cutting concerns polluting the handlers.

A filter could be implementing an IExecutionFilter interface like the following.

public interface IExecutionFilter
{
    Task ExecuteAsync<TCommand>(ICommandContext<TCommand> context, CommandFilterDelegate<TCommand> next) where TCommand : class, ICommand;

    Task ExecuteAsync<TEvent>(IEventContext<TEvent> context, EventFilterDelegate<TEvent> next) where TEvent : class, IEvent;
}

public delegate Task CommandFilterDelegate<T>(ICommandContext<T> context) where T : class, ICommand;

public delegate Task EventFilterDelegate<T>(IEventContext<T> context) where T : class, IEvent;

Here is a sample of a filter

public class SampleExecutionFilter : IExecutionFilter
{
    async Task IExecutionFilter.ExecuteAsync<TCommand> (ICommandContext<TCommand> context, CommandFilterDelegate<TCommand> next)
    {
        // do something before

        await next(context);

        // do something after
    }

    async Task IExecutionFilter.ExecuteAsync<TEvent> (IEventContext<TEvent> context, EventFilterDelegate<TEvent> next)
    {
        // do something before

        await next(context);

        // do something after
    }
}

[Kralizek]

The IExecutionFilter should also intercept the message before it gets pushed to the IBusEngine