Support Cluster API supplementary and primary groups

[navaneeth-dev]

Goal

Preserve Cluster API worker-user group semantics in generated Butane configs.

Required behavior

Supplementary groups

• Convert the comma-separated cloud-config string into Butane's list representation.
• Split on commas and trim surrounding whitespace.
• Preserve source order.
• Reject empty or duplicate group names rather than silently cleaning the input.

primary_group

• Ensure the named primary group exists by emitting the corresponding passwd.groups entry.
• Reference that group from the generated Butane user.
• Deduplicate group declarations shared by multiple users.

Context

Cloud-init creates missing supplementary and primary groups. Butane's user fields only reference groups, so direct field mapping is not semantically complete. This work is deferred from the first users-stanza PR and must preserve all-or-nothing transpilation: unsupported or invalid group input fails conversion instead of being dropped.