bugfix: ApiResponseRatelimitCounter response header handling

[rosskirkpat]

What this PR does / why we need it:

Fixes an edge case bug in handling POST /linode/instances linodego calls where the Linode is successfully created and the api response is missing header(s). This causes the request to fail, with ERROR LinodeMachineReconciler Failed to create Linode machine instance. This is due to the ApiResponseRatelimitCounter method not checking if the X-Ratelimit-Remaining and X-Ratelimit-Reset headers have a non-empty value, which causes the strconv call to fail: "[002] strconv.Atoi: parsing \"\": invalid syntax".

This PR modifies the logic to not return an error and instead to log the error in the case of one or both of the headers not being present in the response. I added default values for each of the headers.
Per https://techdocs.akamai.com/linode-api/reference/rate-limits#specific-operation-rate-limits Creating Linodes has a dedicated rate limit of 20 requests per 15 seconds.

X-Ratelimit-Remaining: 19 (max of 20 per 15 seconds, minus the one just created)
X-Ratelimit-Reset: 15 (seconds)

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 48.14815% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.03%. Comparing base (59420ee) to head (0ce2b04).

Files with missing lines	Patch %	Lines
util/ratelimits.go	48.14%	8 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1070      +/-   ##
==========================================
- Coverage   69.15%   69.03%   -0.12%     
==========================================
  Files          71       71              
  Lines        6543     6563      +20     
==========================================
+ Hits         4525     4531       +6     
- Misses       1724     1732       +8     
- Partials      294      300       +6     

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