Short lived tokens

[aoberoi]

Summary

The culmination of the work to support short-lived tokens in the WebClient.

Fixes #617

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #625 into master will decrease coverage by 0.12%.
The diff coverage is 92.22%.

@@            Coverage Diff             @@
##           master     #625      +/-   ##
==========================================
- Coverage   92.01%   91.89%   -0.13%     
==========================================
  Files           7        7              
  Lines         426      481      +55     
  Branches       78       89      +11     
==========================================
+ Hits          392      442      +50     
- Misses         19       22       +3     
- Partials       15       17       +2

Impacted Files	Coverage Δ
src/methods.ts	100% <ø> (ø)	⬆️
src/errors.ts	100% <100%> (ø)	⬆️
src/WebClient.ts	93.75% <92.13%> (-0.66%)	⬇️

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