fix(Cdn): make parameters immutable

[kyranet]

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

Following the efforts outlined in https://github.com/discordjs/discord.js/issues/5267, this PR modifies the CDN class so the parameters passed are not mutated. Also abstracts the code to reduce duplication and makes types stricter (removes dynamic from those that do not read it), which makes this semver: major (possibly? The behavior doesn't change, TS users just get an error for it).

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[bot]]

Codecov Report

Merging #84 (143e649) into main (7c2b0c0) will increase coverage by 0.11%. The diff coverage is 64.10%.

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
+ Coverage   88.91%   89.02%   +0.11%     
==========================================
  Files           8        8              
  Lines        1371     1376       +5     
  Branches      144      143       -1     
==========================================
+ Hits         1219     1225       +6     
  Misses          9        9              
+ Partials      143      142       -1     

Impacted Files	Coverage Δ
packages/rest/src/lib/CDN.ts	88.83% <64.10%> (+0.71%)	:arrow_up:

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