Add support for locality policies to net-lb-app-ext module

[jacklever-hub24]

This PR fixes the missing locality_lb_policy/policies fields and blocks missing from the net-lb-app-ext module in CFF. It has two validations to prove correctness of incoming arguments but needs an additional test written to validate the correctness (test-plan-llp.yaml).

---

Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

** Upgrade Notes **

`modules/net-lb-app-ext`: Adds the two missing fields for locality_lb_policy and locality_lb_policies with field and block set, validation for both and tests.

[ludoo]

Thanks for this. Can you sign the CLA?

[jacklever-hub24]

Fixes #2882

[ludoo]

Dropped a couple nits, then this is good to go. Thanks! :)

[jacklever-hub24]

I've had to revert one of the null checks as it failed the tests. line 169 - backend_service.locality_lb_policy == null ? true :