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.

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modules/convert/utils.go

@@ -33,6 +33,8 @@ func ToGitServiceType(value string) structs.GitServiceType {
 		return structs.GitlabService
 	case "gogs":
 		return structs.GogsService
+	case "onedev":
+		return structs.OneDevService
 	default:
 		return structs.PlainGitService
 	}