v2/accounting: Implement thin NeoFS gRPC client

Defines thin wrapper over the generated gRPC Accounting service client to
fix some of its options.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

v2/accounting/grpc/client.go

@@ -0,0 +1,62 @@
+package accounting
+
+import (
+	"context"
+
+	"github.com/pkg/errors"
+	"google.golang.org/grpc"
+)
+
+// Client wraps AccountingServiceClient
+// with pre-defined configurations.
+type Client struct {
+	*cfg
+
+	client AccountingServiceClient
+}
+
+// Option represents Client option.
+type Option func(*cfg)
+
+type cfg struct {
+	callOpts []grpc.CallOption
+}
+
+// ErrNilAccountingServiceClient is returned by functions that expect
+// a non-nil AccountingServiceClient, but received nil.
+var ErrNilAccountingServiceClient = errors.New("accounting gRPC client is nil")
+
+func defaultCfg() *cfg {
+	return new(cfg)
+}
+
+// NewClient creates, initializes and returns a new Client instance.
+//
+// Options are applied one by one in order.
+func NewClient(c AccountingServiceClient, opts ...Option) (*Client, error) {
+	if c == nil {
+		return nil, ErrNilAccountingServiceClient
+	}
+
+	cfg := defaultCfg()
+	for i := range opts {
+		opts[i](cfg)
+	}
+
+	return &Client{
+		cfg:    cfg,
+		client: c,
+	}, nil
+}
+
+func (c *Client) Balance(ctx context.Context, req *BalanceRequest) (*BalanceResponse, error) {
+	return c.client.Balance(ctx, req, c.callOpts...)
+}
+
+// WithCallOptions returns Option that configures
+// Client to attach call options to each rpc call.
+func WithCallOptions(opts []grpc.CallOption) Option {
+	return func(c *cfg) {
+		c.callOpts = opts
+	}
+}