Fix #868 Add ability to use a custom express app and/or router to ExpressReceiver

[seratch]

Summary

This pull request resolves #868. With this change, developers can easily embed Bolt app endpoints in existing Express applications.

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 #1084 (8975e85) into main (02d601d) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1084      +/-   ##
==========================================
+ Coverage   68.73%   68.79%   +0.05%     
==========================================
  Files          13       13              
  Lines        1206     1208       +2     
  Branches      355      359       +4     
==========================================
+ Hits          829      831       +2     
  Misses        304      304              
  Partials       73       73              

Impacted Files	Coverage Δ
src/receivers/ExpressReceiver.ts	66.66% <100.00%> (+0.41%)	⬆️

---

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 02d601d...8975e85. Read the comment docs.

[mwbrooks]

🎉 Thanks for adding this!

[filmaj]

Looks good!

What, if any, documentation additions would be needed for this? Interestingly, there seems to be some discussion at least around the router property included in this PR on the custom routes docs - and it even says that that's been a feature since 2.1!? Something doesn't add up here...

[seratch]

@filmaj

What, if any, documentation additions would be needed for this?

We can update the custom routes document to use HTTPReceiver(current default receiver) once we release #866 (@misscoded is working on it now). I will create a new task issue for it.