MM-43144: Add teams loader (#19960)

```release-note
NONE
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/resolver.go

@@ -303,3 +303,12 @@ func getChannelsLoader(ctx context.Context) (*dataloader.Loader, error) {
 	}
 	return l, nil
 }
+
+// getTeamsLoader returns the teams loader out of the context.
+func getTeamsLoader(ctx context.Context) (*dataloader.Loader, error) {
+	l, ok := ctx.Value(teamsLoaderCtx).(*dataloader.Loader)
+	if !ok {
+		return nil, errors.New("no dataloader.Loader found in context")
+	}
+	return l, nil
+}