Add event processor functionality

[caaavik-msft]

This addresses #2054 which is a feature request to have hooks which can enable other tools that are invoking BDN to display progress rather than having to parse live console output or to wait for all the benchmarks to complete to have the exporters run.

A full proposal and explanation of the events/hooks in this PR are covered in this comment: #2054 (comment)

There are potentially many more hooks that could be added like events for when a benchmark run enters a different stage, but they would likely need to be added as custom events sent through the Broker and supported by all the different executors/toolchains, which I didn't want to add at this stage. I'm open as well to modifying or adding extra events before merging this in to see what people think would be useful.

I've also included a EventHandlerBase class which is a base class that implements all the hooks as virtual methods, so that implementing classes only need to override the hooks they care about.

[cincuranet]

Any reason why CompositeEventHandler does not derive from EventHandlerBase?

[caaavik-msft]

Any reason why CompositeEventHandler does not derive from EventHandlerBase?

EventHandlerBase is just an implementation of the interface with virtual methods so that custom event handlers don't have to implement every hook and can just implement the hooks they care about. The CompositeEventHandler composes multiple event handlers into one, so by nature we want to ensure that every hook is implemented and handled, if we used the EventHandlerBase we may forget to update CompositeEventHandler if we add a new hook.

[adamsitnik]

Overall it looks good, but let's iterate on simplifying it a bit and unifying the naming.

Thank you for your contribution @caaavik-msft !

[adamsitnik]

We are almost there, thank you for addressing the feedback @caaavik-msft !

[caaavik-msft]

@adamsitnik are we good to merge this in now?

[adamsitnik]

LGTM, but please rename the type before I hit the merge button.

Thank you @caaavik-msft !

[adamsitnik]

what is the difference between OnBuildComplete and OnEndBuildStage? The first is for one partition and the latter is for all of them?

[caaavik-msft]

Yes, that is the difference between them.

[mawosoft]

@adamsitnik, @caaavik-msft

nit: it's unlikely that the users will need more than one instance

While this may be true, this makes it the only one of the Add... methods that doesn't accept a params array. Personally, I would prefer the API consistency.

[mawosoft]

Also, ManualConfig.Add(IConfig config) and, by extension, ManualConfig.Union(IConfig globalConfig, IConfig localConfig) and ManualConfig.Create(IConfig config) don't copy the event processor(s) from the source.