bgp: T6473: missing completion helper for peer-groups inside a VRF (backport #3638)

[mergify]

Change Summary

Using BGP peer-groups inside a VRF instance will make use if the global VRFs peer-group list during tab-completion and not the peer-groups defined within the BGP instance of the given VRF.

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/T6473

Related PR(s)

Component(s) name

bgp

Proposed changes

How to test

set vrf name red protocols bgp neighbor 1.2.3.4 peer-group 'PG-VYOS-123'
set vrf name red protocols bgp neighbor 100.64.151.254 peer-group 'PG-VYOS-123'
set vrf name red protocols bgp peer-group PG-VYOS remote-as 'external'
set vrf name red protocols bgp peer-group PG-VYOS-123 address-family ipv4-unicast nexthop-self
set vrf name red protocols bgp peer-group PG-VYOS-123 remote-as 'internal'
set vrf name red protocols bgp system-as '2000'
set vrf name red table '2000'

vyos@vyos# set vrf name red protocols bgp neighbor 10.10.10.10 peer-group
Possible completions:
   <text>               Peer-group name
   PG-VYOS
   PG-VYOS-123

Smoketest result

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

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This is an automatic backport of pull request #3638 done by [Mergify](https://mergify.com).

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