feat(CDN): add role icon endpoint

[iShibi]

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

Adds roleIcon method to generate URL for a role's icon

• Document ROLE_ICONS discord/discord-api-docs#3847

Status and versioning classification:

• I know how to update typings and have done so
• This PR changes the library's interface (methods or parameters added)

[codecov]

Codecov Report

Merging #64 (fdb3f6b) into main (c4b2803) will increase coverage by 0.04%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
+ Coverage   85.80%   85.84%   +0.04%     
==========================================
  Files           8        8              
  Lines        1000     1010      +10     
  Branches      100      101       +1     
==========================================
+ Hits          858      867       +9     
  Misses         42       42              
- Partials      100      101       +1     

Impacted Files	Coverage Δ
packages/rest/src/lib/CDN.ts	86.74% <90.00%> (+0.20%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c4b2803...fdb3f6b. Read the comment docs.