Notification feature

[KJ202]

This correponds to the first PR implementing the Notification feature.
We implement here the smtp notification. The SMTP provider should support an API key to allow the feature to work properly (it has been tested with gmail).

The tricky part for me right now is the unit test implementation with our current framework.
To sum up:
I added a new protocol smtp.
Then, I naturally added the send_email function in smtp.go.
The problem is that this function requires API keys. I decided to allow the user to provide them through the config file. This does not seem to match our current model where the config file is mainly used for user sources API keys though. Besides, the testing framework provides a setupconfig function which is also only accessible in the services package.

So, right now, the quickest solution to start implemeting my tests would be to import the services package in my smtp_test.go file. if you agree, I'll start to work on it this way.
Otherwise, I'm open to any suggestion to get a better solution.

PS: This PR is still work in progress. I still need to handle 2 more cases and finish the tests to be able to set it as Ready.

[caffix]

Let's continue the implementation that includes the other methods of notification