[improvement] [breaking] make subnetID optional and align new interface field names with existing field names

[eljohnson92]

What this PR does / why we need it:
This PR aligns the new linode interfaces spec with the legacy interfaces spec. it does include some breaking changes in the new currently unused spec
Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

TODOs:

• squashed commits
• includes documentation
• adds unit tests
• adds or updates e2e tests

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.20%. Comparing base (06cd6e6) to head (038ae61).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...nal/controller/linodemachine_controller_helpers.go	40.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #880      +/-   ##
==========================================
- Coverage   66.22%   66.20%   -0.03%     
==========================================
  Files          71       71              
  Lines        7959     7963       +4     
==========================================
+ Hits         5271     5272       +1     
- Misses       2389     2391       +2     
- Partials      299      300       +1     

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