WIP: types v3 message attachment breaking changes

[filmaj]

fixes #1816

• Use of discriminated unions
• type tests fully describing constraints

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.26%. Comparing base (a2ba1c1) to head (1d4796c).

Additional details and impacted files

@@            Coverage Diff            @@
##           types-v3    #1911   +/-   ##
=========================================
  Coverage     90.26%   90.26%           
=========================================
  Files            34       34           
  Lines          7666     7666           
  Branches        381      381           
=========================================
  Hits           6920     6920           
  Misses          734      734           
  Partials         12       12           

Flag	Coverage Δ
cli-hooks	94.94% <ø> (ø)
cli-test	96.93% <ø> (ø)
oauth	76.53% <ø> (ø)
socket-mode	59.59% <ø> (ø)
web-api	96.57% <ø> (ø)
webhook	95.27% <ø> (ø)

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