This belongs to the OAuth Epic: #4004 - please read this for the big picture of what this issue is for :)

Requires #4176

The following API endpoints are needed to make BREAD operations available for clients. These methods are needed for implementing the client management screen (see #4177). The endpoints are very similar to our other implementations and should follow the best practice used there.

• GET /ghost/v0.1/clients
• GET /ghost/v0.1/clients/<id>
• GET /ghost/v0.1/clients/slug/<slug>
• POST /ghost/v0.1/clients/
• DELETE /ghost/v0.1/clients/<id>
• PUT /ghost/v0.1/clients/<id>

Expected response format:

{
    clients:[
        {...}
    ]
}