New component-retrieved event

[matteofigus]

This is needed for having insight about each component, especially when consuming the registry via the new Batch POST endpoint: #163

[matteofigus]

• I made some cleanup on the code, just for consistency
• The event handler is now a singleton module - this allows it to be just required in multiple places for firing events without inject the instance on all the dependent classes, still keeping all the subscriptions. Tests attached
• The request event contains the body - useful for the new POST endpoint
• The component-retrieved event is fired and contains information about the component. Tests attached.

@jankowiakmaria can you review?

I apology if it is a big PR containing many things - should have done multiple ones.
Thanks

[jankowiakmaria]

LGTM - should I merge (the title of PR suggests that not yet)? @matteofigus

[matteofigus]

Yes, sorry