feat: Add SequenceBottomNavigationSlot for customizable sequence navigation

[xitij2000]

Introduces a new plugin slot (SequenceBottomNavigationSlot) for extending or replacing the default bottom navigation in course sequences.

[openedx-webhooks]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.43%. Comparing base (1ea080f) to head (4924cad).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1918      +/-   ##
==========================================
+ Coverage   91.30%   91.43%   +0.12%     
==========================================
  Files         344      347       +3     
  Lines        5774     5790      +16     
  Branches     1385     1390       +5     
==========================================
+ Hits         5272     5294      +22     
+ Misses        483      477       -6     
  Partials       19       19              

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[xitij2000]

@brian-smith-tcril I've applied all your suggestions. This is ready for another look.

[brian-smith-tcril]

This is 99% ready to go! Just a couple README updates and I'll be happy to land it!