Implement new request signing mechanism

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

util/signature/util.go

@@ -0,0 +1,31 @@
+package signature
+
+import (
+	"sync"
+
+	"github.com/pkg/errors"
+)
+
+var bytesPool = sync.Pool{
+	New: func() interface{} {
+		return make([]byte, 5<<20)
+	},
+}
+
+func dataForSignature(src DataSource) ([]byte, error) {
+	if src == nil {
+		return nil, errors.New("nil source")
+	}
+
+	buf := bytesPool.Get().([]byte)
+
+	if size := src.SignedDataSize(); size < 0 {
+		return nil, errors.New("negative length")
+	} else if size <= cap(buf) {
+		buf = buf[:size]
+	} else {
+		buf = make([]byte, size)
+	}
+
+	return src.ReadSignedData(buf)
+}