Add `Conn` method to `Client` interface which must return the underlying
connection. Implement new method on the core structure. `Conn` can be used
to control the connection (e.g. for closing).
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Make `Client` to be the wrapper over raw protobuf client. Provide public
method to get the underlying raw client. Change implementations of all
methods with the new approach of the RPC execution.
Additional changes:
* key replaced from `New` argument to `WithDefaultPrivateKey` option;
* `GetSelfBalance` is removed as non-viable;
* `GetEACLWithSignature` is removed, `GetEACL` returns `EACLWithSignature`;
* `AttachSessionToken` / `AttachBearerToken` are removed as non-viable;
* redundant options are removed.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
