Signal Statistic endpoint

[meschac38700]

We should be able to list the number of signals registered in the current instance of the signal manager.

• Number of signals
• Number of signals filtered by name
• Number of signals filtered by application
• Number of signals filtered by types (Mapper, Schema, Session etc...)

This is an example of a response to be completed during development

{
    "total_signals": 4,
    "total_listeners": 10,
    "signals": {
        "before_update": 4,
        "after_update": 2,
        "before_delete": 3,
        "before_insert": 1
    },
    "details": [
        {
            "name": "before_update",
            "target": "apps.user.models.user.User",
            "callback": "function name  or path to the function"
        }
    ]
}

Depends on #62