There is a need to generalize single-address client to group-address client.
To do this, we can re-implement `Client` interface from NeoFS API Go library
and still use it in the application code. There is a problem with method
`Raw` which must return single-address raw client. So as not to make changes
to API library we need to overload Client interface in order to support
`Raw` method in group-address client implementation.
Define `Client` interface in new `pkg/core/client` package. Completely
inherit API `Client` interface. Add `RawForAddress` method to build raw
client for the single node address. Adopt the application code that used Raw
method to work with new `Client`.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
