fix: default flag parser for apps (backport #4483)

[mergify]

Fix default values for app flags, it always gets zero if the value is not set

---

This is an automatic backport of pull request #4483 done by Mergify.

[mergify]

Cherry-pick of 7f91423 has failed:

On branch mergify/bp/release/v28.x.y/pr-4483
Your branch is up to date with 'origin/release/v28.x.y'.

You are currently cherry-picking commit 7f914230.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   changelog.md
	modified:   ignite/services/plugin/flag.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   ignite/services/plugin/flag_test.go
	both modified:   ignite/services/plugin/grpc/v1/interface_flag.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally