adding IncomingWebhookResult to exports

[mledom]

Summary

IncomingWebhookResult was not currently exported.

Requirements (place an x in each [ ])

• [x ] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x ] I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #562 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #562   +/-   ##
=======================================
  Coverage   81.72%   81.72%           
=======================================
  Files           6        6           
  Lines         279      279           
  Branches       43       43           
=======================================
  Hits          228      228           
  Misses         36       36           
  Partials       15       15

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[aoberoi]

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