Add `subnet` command which contains all subnet-related commands. Add
sub-commands:
* `create` for creation;
* `remove` for removal;
* `get` for reading;
* `admin` for admin management;
* `client` for client management.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Define notification events, implement parsers. Add morph client of
Subnet contract. Listen, verify and approve events in Inner Ring app.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
