Implement `message.Message` interface on all structures and use new methods
for conversion instead of functions. make `Unmarshal` and JSON methods to
use encoding functions from `message` library. Remove all per-service
clients and implement `rpc` library of the functions which execute NeoFS API
RPC through new RPC client. Remove no longer used gRPC per-service clients.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>
Client constructor `New` will be used as `package.New()`
in external packages. This definition is not very clear
since it can create new structure or new client or whatever.
`package.NewClient()` is quite unambiguous.
Signed-off-by: Alex Vanin <alexey@nspcc.ru>
