fix: correct gitignore pattern to only exclude root binary

Changed 'lambda' to '/lambda' in .gitignore to prevent accidentally
hiding files in pkg/lambda/ directory. The pattern now correctly
excludes only the compiled binary at the repository root.

Also adds previously hidden pkg/lambda/iterator.go to the repository.

.gitignore

@@ -3,7 +3,7 @@
 # https://github.com/github/gitignore/blob/main/community/Golang/Go.AllowList.gitignore
 #
 # Binaries for programs and plugins
-lambda
+/lambda
 *.exe
 *.exe~
 *.dll