Have storescp server support plugins

[aachick]

Is your feature request related to a problem? Please describe.
It would be nice to have the storescp function support "plugin" type functions.
The idea would be to be able to pass functions or string imports to the StoreSCPServer class so that incoming DICOM events can be passed to the functions before the DICOM file gets persisted. This would allow for greater flexibility in how the code can be used.

Describe the solution you'd like
I would like to see an optional StoreSCPServer constructor parameter be introduced. This would allow passing a function or list of functions that would then be run over incoming Event objects. The possibility of passing functions to StoreSCPServer instances should be possible via the configuration file.

Describe alternatives you've considered
I have considered not proposing this feature and letting users implement their own features. However, this probably would result in a lot of biolerplate code.

Additional context

[aachick]

I think that in addition to adding callback functions to the StoreSCPServer server class, the handlers param should be removed. This would limit user errors (by passing weird functions or by registering the functions to the wrong event type).

The question I have is whether to pass only the DICOM to the callback to the function or pass the Event and whether I should include the file path of the persisted DICOM file.