feat(CDN#guildIcon): implement dynamic logic

[Fyko]

Please describe the changes this PR makes and why it should be merged:

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)

[codecov]

Codecov Report

Merging #53 (21bea12) into main (ceaab7d) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
- Coverage   85.84%   85.80%   -0.05%     
==========================================
  Files           8        8              
  Lines         996     1000       +4     
  Branches       99      100       +1     
==========================================
+ Hits          855      858       +3     
  Misses         42       42              
- Partials       99      100       +1     

Impacted Files	Coverage Δ
packages/rest/src/lib/CDN.ts	86.53% <66.66%> (-0.31%)	⬇️

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[kyranet]

I was going to comment about me not liking this mutating pattern, but seems it's used in other methods, so I'll do a PR after this one to refactor that.

[iCrawl]

This needs a rebase @Fyko