Creation of a new protocol for notifications

### Background

Allow user to register and configure Web Push Notifications API clients into Mediator so Mediator can send notifications directly.

Since the App Developer and Mediator are developed by different people we have to create a standard that allow Mediator to receive the URL and Web Token.

This will facilitate integrations with other systems (outside of DID Comm). Webhooks would be very valuable here.
For example, the [Home Assistant](https://www.home-assistant.io/) is used to create advanced automation in your home. It's usually set up on an internal Network due to security concerns.
Communication with agents outside of the internal Network could be done over DID Comm, and Webhooks would make integration with the Home Assistant very easy.

The Support Live Mode in the Pickup Protocol (https://github.com/input-output-hk/atala-prism-mediator/issues/145) helps mitigate some use cases.
But fundamentally the use case is different.
Furthermore, this even complements the pickup protocol because you can push a notification to an agent the other technologies like Web PUSH API.
Especially useful when the agent is a mobile agent (ex: Android, iOS, Browser) and is offline from PoV of the Mediator.

### Definition of Done

Protocol must be able to receive the URL and Token for the Web Push Notification service (interation number 2 in the image)
Protocol must be registered in https://didcomm.org/
Mediator must use the provided data to send notifications directly to client

### Implementation details

This would be integration outside of DID Comm. Like webhooks or the web [Push API](https://developer.mozilla.org/en-US/docs/Web/API/Push_API)

We are talking about a protocol to configure notification channels and conditions.
So services, for example, the Mediator sends notifications if a certain condition is achieved.