MudDataGrid: Respect indeterminate state in select-all checkboxes

[Qwertyluk]

Description

Resolves #6553

How Has This Been Tested?

Added a unit test to ensure that a state (including indeterminate state) of select-all checkboxes change correctly based on the number of selected items in the data grid.
Also conducted manual testing of the changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• The PR is submitted to the correct branch (dev).
• My code follows the code style of this project.
• I've added relevant tests.

[codecov]

Codecov Report

Attention: Patch coverage is 92.30769% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 89.50%. Comparing base (0dcc84c) to head (9161807).

Files	Patch %	Lines
src/MudBlazor/Components/DataGrid/HeaderContext.cs	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           dev    #8317       +/-   ##
========================================
+ Coverage     0   89.50%   +89.50%     
========================================
  Files        0      411      +411     
  Lines        0    11848    +11848     
  Branches     0     2362     +2362     
========================================
+ Hits         0    10604    +10604     
- Misses       0      715      +715     
- Partials     0      529      +529     

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[tjscience]

This looks good. Appreciate it, @Qwertyluk!

[ScarletKuro]

Added to v7.0.0 Migration Guide #8447
since this PR changed public api type from bool to bool?.

[ScarletKuro]

@Qwertyluk thanks.