Update Milestone struct (#410)

Update Milestone struct
add Created and Updated

source: https://github.com/go-gitea/gitea/pull/12662

Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://gitea.com/gitea/go-sdk/pulls/410
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Andrew Thornton <art27@cantab.net>

gitea/issue_milestone.go

@@ -21,6 +21,8 @@ type Milestone struct {
 	State        StateType  `json:"state"`
 	OpenIssues   int        `json:"open_issues"`
 	ClosedIssues int        `json:"closed_issues"`
+	Created      time.Time  `json:"created_at"`
+	Updated      *time.Time `json:"updated_at"`
 	Closed       *time.Time `json:"closed_at"`
 	Deadline     *time.Time `json:"due_on"`
 }