[v2] Add checkboxes action payload

[seratch]

Summary

This pull request adds #408 changes to the v2 branch. I just cherry-picked the commit and it doesn't have any conflict with this branch.

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 #409 into @slack/bolt@next will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           @slack/bolt@next     #409   +/-   ##
=================================================
  Coverage             85.37%   85.37%           
=================================================
  Files                     7        7           
  Lines                   506      506           
  Branches                144      144           
=================================================
  Hits                    432      432           
  Misses                   49       49           
  Partials                 25       25

---

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 f7ca14a...ca7b6ed. Read the comment docs.