firewall: T8089: "geoip country-code" should get a completion helper

[c-po]

Change summary

There is a CLI constraint for lowercase country codes, but user's do not see this.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T8089

How to test / Smoketest result

vyos@vyos# set firewall ipv4 name foo rule 10 source geoip country-code
Possible completions:
   <country>            Country code (2 characters, lowercase)
   ad
   ae
   af
   ...

vyos@vyos# set firewall ipv6 name foo rule 10 source geoip country-code
Possible completions:
   <country>            Country code (2 characters, lowercase)
   ad
   ae
   af
   ...

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

👍
No issues in PR Title / Commit Title

[github-actions]

CI integration ❌ failed!

Details

CI logs

• CLI Smoketests (no interfaces) 👍 passed
• CLI Smoketests VPP 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests ❌ failed
• Config tests VPP 👍 passed
• RAID1 tests 👍 passed
• TPM tests 👍 passed

[sever-sever]

Add completion help for the geip country codes.

[sarthurdev]

Adds completion helper for country-code values. Works as expected.

[dmbaturin]

I was wondering about the IO code since the isles are being transferred to Mauritius, but it's not done yet so there's no reason to exclude the country code.