feat: add MainAppSlot for chatbot plugin

[awais-ansari]

INF-1573

Description

• This PR adds MainAppSlot, allowing a plugin at the root level. For example the chatbot plugin.
• This PR removes the automatic use of the Zendesk component from the App level. The Zendesk logic still lives in the src/common-components directory.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.57%. Comparing base (b41fca3) to head (a45503f).
Report is 31 commits behind head on 2u-main.

Additional details and impacted files

@@             Coverage Diff             @@
##           2u-main    #1320      +/-   ##
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+ Coverage    86.56%   86.57%   +0.01%     
===========================================
  Files          133      134       +1     
  Lines         2411     2413       +2     
  Branches       669      668       -1     
===========================================
+ Hits          2087     2089       +2     
  Misses         311      311              
  Partials        13       13              

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[jsnwesson]

This looks great so far! One thing that I would request is that in your /plugin-slots directory if you can follow the style of documentation that is in Learner Dashboard and Learning's /plugin-slots directories.
Both also include a section in the main README that plugin slots exist and points to where they can be found. That would be useful to include here as well.

[jsnwesson]

Oh that's riiight, this is only a PR that merges into the forked 2u-main branch. Disregard my request unless you think it'll be useful for your own team. If there's a future case where this PR will eventually be pushed up to the master branch, then I think that documentation will have a stronger ask.