Expose cloud accounts management endpoints

[mlabouardy]

Problem

Currently, the onboarding wizard feature isn't connected to the backend. The goal of this PR is to expose the needed endpoints to plug the workflow.

Solution

I've exposed additional API endpoints to manage the onboarding and cloud accounts.

Changes Made

• Exposed the following endpoints:

GET /is_onboarded
GET /cloud_accounts
POST /cloud_accounts
DELETE /cloud_accounts/{id}
PUT /cloud_accounts/{id}

How to Test

To know if the onboarding wizard should be shown or not, the following API endpoint can be requested:
GET /is_onboarded
It returns the following payload:

{
   "onboarded": true
}

To list all connected cloud accounts:
GET /cloud_accounts
It returns the following response:

[
    {
        "id": 1,
        "provider": "AWS",
        "name": "Sandbox",
        "credentials": {
            "path": "/usr/bin/credentials",
            "profile": "default",
            "source": "CREDENTIALS_FILE"
        }
    },
    {
        "id": 2,
        "provider": "DigitalOcean",
        "name": "Production",
        "credentials": {
            "token": "TOKEN"
        }
    }
]

To add a cloud account:
POST /cloud_accounts
It expects the following payload:

[
    {
        "provider": "AWS",
        "name": "Staging",
        "credentials": {
            "path": "/usr/bin/credentials",
            "profile": "default",
            "source": "CREDENTIALS_FILE"
        }
    }
]

To delete a cloud account:
DELETE /cloud_accounts/{id}

To update a cloud account:
PUT /cloud_accounts/{id}

Checklist

• Code follows the contributing guidelines
• Changes have been thoroughly tested
• Documentation has been updated, if necessary
• Any dependencies have been added to the project, if necessary