[#263] pkg/client: Refactor the client to use raw protobuf client

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>

pkg/client/raw.go

@@ -0,0 +1,14 @@
+package client
+
+import (
+	"github.com/nspcc-dev/neofs-api-go/rpc/client"
+)
+
+// Raw returns underlying raw protobuf client.
+func (c *clientImpl) Raw() *client.Client {
+	c.onceInit.Do(func() {
+		c.raw = client.New(c.opts.rawOpts...)
+	})
+
+	return c.raw
+}