add support for cache reserve

[jfchevrette]

Description

Add support for retrieving and updating the Cache Reserve setting on a zone

This is a rather naive implementation which follows the same pattern as the argo.go methods

Closes #1277

Has your change been tested?

Unit tests included

I also tested locally on a real account and I was able to retrieve and set the cache_reserve setting

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[github-actions]

changelog detected ✅

[codecov-commenter]

Codecov Report

Merging #1278 (cf0ce93) into master (3508050) will decrease coverage by 0.02%.
The diff coverage is 43.75%.

@@            Coverage Diff             @@
##           master    #1278      +/-   ##
==========================================
- Coverage   49.07%   49.05%   -0.02%     
==========================================
  Files         133      134       +1     
  Lines       13032    13064      +32     
==========================================
+ Hits         6395     6409      +14     
- Misses       5143     5155      +12     
- Partials     1494     1500       +6     

Impacted Files	Coverage Δ
rulesets.go	34.64% <ø> (ø)
cache_reserve.go	43.75% <43.75%> (ø)

[github-actions]

This functionality has been released in v0.68.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!