feat: Add slots to add tab links and add mechanism for plugin routes

[xitij2000]

Backport of openedx#1645

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 88.83%. Comparing base (8356505) to head (fc20c94).
Report is 5 commits behind head on opencraft-release/redwood.1.

Files with missing lines	Patch %	Lines
src/index.jsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           opencraft-release/redwood.1      #77      +/-   ##
===============================================================
+ Coverage                        88.82%   88.83%   +0.01%     
===============================================================
  Files                              306      310       +4     
  Lines                             5280     5295      +15     
  Branches                          1311     1345      +34     
===============================================================
+ Hits                              4690     4704      +14     
- Misses                             574      575       +1     
  Partials                            16       16              

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