feat: multi point plan useq widgets

[tlambert03]

I'm pulling out the widgets that create multi-points plans from #308, for ease of discussion and testing

[codecov]

Codecov Report

Attention: Patch coverage is 90.50279% with 17 lines in your changes missing coverage. Please review.

Project coverage is 90.03%. Comparing base (6d088a7) to head (9ed073e).
Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
...useq_widgets/points_plans/_random_points_widget.py	88.88%	9 Missing ⚠️
...eq_widgets/points_plans/_grid_row_column_widget.py	89.87%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
+ Coverage   90.01%   90.03%   +0.02%     
==========================================
  Files          67       70       +3     
  Lines        7681     7837     +156     
==========================================
+ Hits         6914     7056     +142     
- Misses        767      781      +14     

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[tlambert03]

@fdrgsp, you can merge this one after having a look at it, then resolve conflicts with #308. Note that i made the two widgets regular QWidgets here, so you should insert them into a QGroupBox inside your FOV widget if you want them to have a border

(it's ok to leave in the temporary dependency on useq-schema main)