To add a new credential use the set_credential api by sending this json
object:

{
    "msg": "set_credential",
    "data": {
        "service": "the new/existing context",
        "login": "the new or existing login",
        "password": "the password",
        "description": "the optional description"
    }
}

This api is used for adding and for updating an existing credential. For
now the description field is ignored as there is no HID command for
changing it yet.