CORS Config API

[mthenw]

What did you implement:

Closes #449

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: NO
Is it a breaking change?: NO

[codecov]

Codecov Report

Merging #460 into master will increase coverage by 2.05%.
The diff coverage is 84.55%.

@@            Coverage Diff             @@
##           master     #460      +/-   ##
==========================================
+ Coverage   66.17%   68.22%   +2.05%     
==========================================
  Files          32       34       +2     
  Lines        1883     2112     +229     
==========================================
+ Hits         1246     1441     +195     
- Misses        585      611      +26     
- Partials       52       60       +8

Impacted Files	Coverage Δ
httpapi/config.go	0% <0%> (ø)	⬆️
internal/cache/target.go	0% <0%> (ø)	⬆️
internal/cache/subscription_cache.go	86.76% <100%> (ø)	⬆️
router/router.go	64.89% <100%> (+1.61%)	⬆️
httpapi/httpapi.go	79.14% <100%> (+6%)	⬆️
httpapi/metrics.go	100% <100%> (ø)	⬆️
libkv/subscription.go	88.6% <100%> (ø)	⬆️
internal/cache/cors_cache.go	75% <75%> (ø)
libkv/cors.go	89.61% <89.61%> (ø)
internal/pathtree/tree.go	97.08% <93.33%> (-0.14%)	⬇️
... and 2 more

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