Add Oauth with ExpressReceiver example app

[srajiang]

Summary

Fixes #715 by adding an example app for Oauth with ExpressReceiver.

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 #1129 (5f76742) into main (ee18b1c) will increase coverage by 1.60%.
The diff coverage is 88.57%.

❗ Current head 5f76742 differs from pull request most recent head d457c00. Consider uploading reports for the commit d457c00 to get more accurate results

@@            Coverage Diff             @@
##             main    #1129      +/-   ##
==========================================
+ Coverage   69.31%   70.92%   +1.60%     
==========================================
  Files          13       14       +1     
  Lines        1284     1324      +40     
  Branches      380      392      +12     
==========================================
+ Hits          890      939      +49     
+ Misses        320      311       -9     
  Partials       74       74              

Impacted Files	Coverage Δ
src/receivers/SocketModeReceiver.ts	84.93% <85.36%> (+17.71%)	⬆️
src/receivers/HTTPReceiver.ts	34.87% <85.71%> (+2.59%)	⬆️
src/receivers/custom-routes.ts	94.44% <94.44%> (ø)
src/App.ts	83.60% <100.00%> (+0.04%)	⬆️
src/errors.ts	100.00% <100.00%> (ø)

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[seratch]

Thanks! LGTM 👍