Add migrate from OneDev (#16356)

* Use context to simplify logic.

* Added migration from OneDev.
This PR adds [OneDev](https://code.onedev.io/) as migration source.

Supported:
- [x] Milestones
- [x] Issues
- [x] Pull Requests
- [x] Comments
- [x] Reviews
- [x] Labels

modules/migrations/base/issue.go

@@ -7,6 +7,25 @@ package base
 
 import "time"
 
+// IssueContext is used to map between local and foreign issue/PR ids.
+type IssueContext interface {
+	LocalID() int64
+	ForeignID() int64
+}
+
+// BasicIssueContext is a 1:1 mapping between local and foreign ids.
+type BasicIssueContext int64
+
+// LocalID gets the local id.
+func (c BasicIssueContext) LocalID() int64 {
+	return int64(c)
+}
+
+// ForeignID gets the foreign id.
+func (c BasicIssueContext) ForeignID() int64 {
+	return int64(c)
+}
+
 // Issue is a standard issue information
 type Issue struct {
 	Number      int64
@@ -25,4 +44,5 @@ type Issue struct {
 	Labels      []*Label
 	Reactions   []*Reaction
 	Assignees   []string
+	Context     IssueContext `yaml:"-"`
 }