Extpillar nodegroups support lists

[oeko2002]

What does this PR do?

Supporting of YAML lists for the nodegroups ext_pillar

What issues does this PR fix or reference?

#55146

Previous Behavior

Nodegroups formed as YAML list

nodegroups:
  mynodegroup:
    - host1
    - host2

fail to parse as ext_pillar nodegroups, resulting in ERROR logs:

2026-05-06 12:47:54,667 [salt.utils.minions:616 ][DEBUG   ][528] Evaluating final compound matching expr: {'host1'} {'host2'} 
2026-05-06 12:47:54,668 [salt.utils.minions:621 ][ERROR   ][528] Invalid compound target: ['host1', 'host2']

New Behavior

Parsing works:

2026-05-06 12:47:55,097 [salt.utils.minions:616 ][DEBUG   ][528] Evaluating final compound matching expr: {'host1', 'host2'}
2026-05-06 12:47:55,100 [salt.utils.minions:482 ][DEBUG   ][528] minions: {'host1', 'host2'}

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

No

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[dwoz]

@oeko2002 This needs to go into 3006.x. I've took the liberty of changing the base branch but there are conflicts that need to be resolved.

[oeko2002]

Ok, rebased to 3006.x branch :D