Expose useful functions

[WilliamBergamin]

Summary

The goal of this PR is to export some helper function that could be useful for developers implementing their own authorization flow.

This resolves #1950

A user will be able to use these function as follows

const { isBodyWithTypeEnterpriseInstall, isEventTypeToSkipAuthorize } = require("@slack/bolt/dist/helpers");

Note: these helper functions are not exported from the index.ts file and should be considered unstable by the end user (minimal breaking change is expected for these functions in the future)

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 #1955 (133c89d) into main (51ee0b5) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1955      +/-   ##
==========================================
+ Coverage   82.18%   82.21%   +0.03%     
==========================================
  Files          18       18              
  Lines        1521     1524       +3     
  Branches      436      438       +2     
==========================================
+ Hits         1250     1253       +3     
  Misses        175      175              
  Partials       96       96              

Files Changed	Coverage Δ
src/App.ts	83.41% <100.00%> (-0.42%)	⬇️
src/helpers.ts	85.71% <100.00%> (+4.76%)	⬆️

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