Change `shared_statistics` property of the network service config to allow "policies"

For now the shared_statistics property is an object, with a single attribute nsc_available_capacity, which is a list of account IDs allowed to access the statistics.

Proposal:

Change into

{
   "policy": "allow"
   "accounts_denied": []
}

{
   "policy": "deny",
   "accounts_allowed": []
}