[fix] getExistingSharedIPs should get Public IPs from the holder

[dthorsen]

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Description

Calls to getExistingSharedIPs should return Public IPs instead of Shared IPs, because the IPs on the holder nanode are not returned under the Shared IP's section of the API response.

fixes #231

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.29%. Comparing base (708ee98) to head (0568585).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
+ Coverage   55.34%   56.29%   +0.95%     
==========================================
  Files          12       12              
  Lines        1881     1881              
==========================================
+ Hits         1041     1059      +18     
+ Misses        696      672      -24     
- Partials      144      150       +6     

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[AshleyDumaine]

@dthorsen we're looking at moving away from the ip-holder workaround now that linodego has released support for the reserved IPs API endpoints.