fix(migrations): transfer PR flow information (#7421)

- When migrating a pull requests from a Gitea or Forgejo instance, check if the pull request was created via the AGit flow and transfer that bit of information to the migrated pull request.
- Expose this bit of information as the `flow` field for the pull request.
- We have to do a horrible Go hack with Gitea's [go-sdk](gitea.com/gitea/go-sdk) to list all pull requests while being able to decode it to a struct that contains the new `Flow` field. The library does not allow you to do this out of the box, so we have to use `go:linkname` to access the private method that allows us to do this. This in turn means we have to do some boilerplate code that the library otherwise would do for us. The better option would be forking, but that would be a hassle of keeping the library in sync.
- Resolves forgejo/forgejo#5848
- Unit test added.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7421
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

modules/structs/pull.go

@@ -57,7 +57,8 @@ type PullRequest struct {
 	// swagger:strfmt date-time
 	Closed *time.Time `json:"closed_at"`
 
-	PinOrder int `json:"pin_order"`
+	PinOrder int   `json:"pin_order"`
+	Flow     int64 `json:"flow"`
 }
 
 // PRBranchInfo information about a branch