[#1461] cli: Reorganize common functions

Make `ParseEpoch` and `GetCurrentEpoch` funcs public and move the first one
to the `common` package.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

cmd/neofs-cli/internal/common/epoch.go

@@ -0,0 +1,28 @@
+package common
+
+import (
+	"fmt"
+	"strconv"
+
+	"github.com/spf13/cobra"
+)
+
+// ParseEpoch parses epoch argument. Second return value is true if
+// the specified epoch is relative, and false otherwise.
+func ParseEpoch(cmd *cobra.Command, flag string) (uint64, bool, error) {
+	s, _ := cmd.Flags().GetString(flag)
+	if len(s) == 0 {
+		return 0, false, nil
+	}
+
+	relative := s[0] == '+'
+	if relative {
+		s = s[1:]
+	}
+
+	epoch, err := strconv.ParseUint(s, 10, 64)
+	if err != nil {
+		return 0, relative, fmt.Errorf("can't parse epoch for %s argument: %w", flag, err)
+	}
+	return epoch, relative, nil
+}