feat(webhook): add rate limiting to webhook endpoint

[cjcocokrisp]

This PR aims to add rate limiting to the webhook endpoint so it can not be spammed with requests. This is accomplished with a sliding window approach where timestamps of requests are stored and then when you make a request it is checked how many requests were made in the specified window from the time of your request. If the window was fixed you could burst the requests at the end of the window and then at the start of the window which could cause overloading problems. The sliding window approach makes it so this is not a problem. The downside to this approach however, is that it requires more memory to store each individual timestamp. Because container registries could have rotating IP addresses this can be a problem due to information of an IP that won't make any more requests will still be stored. So to fix this I added a clean up interval that can be set that will clean any clients that have not been seen in a while.

I looked into some packages that were available and found the rate and uber/ratelimit package but after doing research into what strategies are used for rate limiting it seemed like it wasn't the most difficult to implement.

I will add the documentation for this in a separate PR.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.26966% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.62%. Comparing base (18767ed) to head (56b3178).

Files with missing lines	Patch %	Lines
cmd/run.go	36.36%	7 Missing ⚠️
cmd/webhook.go	36.36%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1210      +/-   ##
==========================================
+ Coverage   63.05%   63.62%   +0.56%     
==========================================
  Files          23       24       +1     
  Lines        3140     3222      +82     
==========================================
+ Hits         1980     2050      +70     
- Misses       1050     1062      +12     
  Partials      110      110              

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[cjcocokrisp]

Resolved the things mentioned in the review.

[chengfang]

extra _ at the end of the env variable name.

[cjcocokrisp]

Sorry about that, tried to get that done quick before leaving the office fixed again.

[jannfis]

I was wondering why you would roll up your own rate limiter, especially given the fact that we already use the rate limiter from uber within image updater

[cjcocokrisp]

I was wondering why you would roll up your own rate limiter, especially given the fact that we already use the rate limiter from uber within image updater

I did not realize this and haven't really touched the registry scanner. I'll refactor this PR tomorrow thanks for the advice I should definitely look more at the packages used next time 🤦‍♂️