feat(RequestManager): support setting global headers in options

[ckohen]

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

Back on the horse trying to catch up /rest to what exists in the main lib.

This PR ports the logic from

• https://github.com/discordjs/discord.js/pull/5586

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 changes the library's interface (methods or parameters added)

[codecov[bot]]

Codecov Report

Merging #70 (601e809) into main (65cb8e2) will increase coverage by 0.09%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
+ Coverage   85.84%   85.93%   +0.09%     
==========================================
  Files           8        8              
  Lines        1010     1017       +7     
  Branches      101      101              
==========================================
+ Hits          867      874       +7     
  Misses         42       42              
  Partials      101      101              

Impacted Files	Coverage Δ
packages/rest/src/lib/REST.ts	95.58% <100.00%> (+0.12%)	:arrow_up:
packages/rest/src/lib/RequestManager.ts	82.72% <100.00%> (+0.05%)	:arrow_up:
packages/rest/src/lib/utils/constants.ts	100.00% <100.00%> (ø)

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