Add Index to comment for migrations and mirroring (#18806)

Comments have an id (see Gitea[0], GitLab[1], GitHub[2], etc.), and the
comment migration format must represent it during migrations so that
it can be used during mirroring or incremental migrations.

[0] https://try.gitea.io/api/swagger#/issue/issueGetComment
[1] https://docs.gitlab.com/ee/api/discussions.html#get-single-issue-discussion-item
[2] https://docs.github.com/en/rest/reference/issues#get-an-issue-comment

Signed-off-by: Loïc Dachary <loic@dachary.org>
Co-authored-by: Loïc Dachary <loic@dachary.org>

modules/migration/comment.go

@@ -9,7 +9,8 @@ import "time"
 
 // Comment is a standard comment information
 type Comment struct {
-	IssueIndex  int64  `yaml:"issue_index"`
+	IssueIndex  int64 `yaml:"issue_index"`
+	Index       int64
 	PosterID    int64  `yaml:"poster_id"`
 	PosterName  string `yaml:"poster_name"`
 	PosterEmail string `yaml:"poster_email"`