Expose user ID in context

[gilmatok]

Summary

Getting the user ID is as important as getting the team ID. Since every Slack action is mostly performed by a user, and we already have logic to extract the user ID from the event's body, we can expose it on the context object like we expose the team ID.

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.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @gilmatok to sign the Salesforce Inc. Contributor License Agreement.

[seratch]

Thanks for sending this PR; LGTM!

[codecov]

Codecov Report

Merging #1853 (ab80673) into main (a3532c3) will increase coverage by 0.02%.
The diff coverage is 100.00%.

❗ Current head ab80673 differs from pull request most recent head c11997b. Consider uploading reports for the commit c11997b to get more accurate results

@@            Coverage Diff             @@
##             main    #1853      +/-   ##
==========================================
+ Coverage   82.15%   82.18%   +0.02%     
==========================================
  Files          18       18              
  Lines        1519     1521       +2     
  Branches      435      436       +1     
==========================================
+ Hits         1248     1250       +2     
  Misses        175      175              
  Partials       96       96              

Impacted Files	Coverage Δ
src/types/middleware.ts	100.00% <ø> (ø)
src/App.ts	83.82% <100.00%> (+0.07%)	⬆️

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