CodeViewer: Implement HeaderContent

[mckaragoz]

Fix #633

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.38%. Comparing base (12572c0) to head (8363894).

Files with missing lines	Patch %	Lines
...zor.Extensions.Code/Components/MudCodeViewer.razor	50.00%	1 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (50.00%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #640      +/-   ##
==========================================
- Coverage   65.41%   65.38%   -0.03%     
==========================================
  Files         111      111              
  Lines        8379     8381       +2     
  Branches     1479     1480       +1     
==========================================
- Hits         5481     5480       -1     
- Misses       2392     2394       +2     
- Partials      506      507       +1     

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