Support a new "state" property in dialogs

[DominikPalo]

Summary

Allow passing extra information through dialogs using the state property. See: https://api.slack.com/dialogs#state

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 #619 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #619   +/-   ##
=======================================
  Coverage   88.96%   88.96%           
=======================================
  Files           7        7           
  Lines         426      426           
  Branches       74       74           
=======================================
  Hits          379      379           
  Misses         31       31           
  Partials       16       16

Impacted Files	Coverage Δ
src/methods.ts	100% <ø> (ø)	⬆️

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