Retrieve CARP configuration for a specific node

Summary:
Introduce a new helper function get_carp_entries() to retrieve CARP
configuration entries from pillar network data.

The function iterates over interfaces, filters FHRP entries using
the "carp" protocol, and returns structured and ordered data including:

• device (interface name)
• vhid
• vip
• advskew (defaulting to 0 when not specified)

Ref T2264

Test Plan:
Added unit tests for get_carp_entries to validate:

• correct extraction of CARP entries when present, ignoring non-CARP FHRP entries
• proper handling of empty FHRP lists

Executed the tests and confirmed all tests pass

Reviewers: dereckson, Duranzed

Reviewed By: dereckson

Maniphest Tasks: T2264

Differential Revision: https://devcentral.nasqueron.org/D3979