[WIP] feat(http-add-on): Add watchLabel for HTTPScaledObjects

[kahirokunn]

Provide a description of what has been changed

Checklist

• [x] I have verified that my change is according to the deprecations & breaking changes policy
• [x] Commits are signed with Developer Certificate of Origin (DCO - learn more)
• [ ] README is updated with new configuration values (if applicable) learn more
• [ ] A PR is opened to update KEDA core (repo) (if applicable, ie. when deployment manifests are modified)

Fixes #