Implementation of StorageProvider events

[jimmyaxod]

In order to support things like S3 Sync, we need a simple event system to be able to send events from one part of Silo to another, allowing for the fact that StorageProviders may have been wrapped / changed etc by the consumer of Silo.

Example:

data := sources.NewMemory(1024*1024)

// In Silo

storage.AddEventListener(data, "sync_start", callback)

// Silo user code

metrics := modules.NewMetrics(data)

silo.DoSomethingWith(metrics)

// Back to silo

returned_data := storage.SendEvent(metrics, ….)

In this example, the callback registered against the storage "data" should get called when the event is triggered against metrics.

[ShivanshVij]

All the changes look good to me, looks like for the most part you were adding the same function implementation to all of them. Might make sense to check the provider variable in its own struct, implement the function once, and then it'll get inherited by all the structs.

Not worth changing right now, but worth keeping in mind.