CMCL-1591: Add LookAtDataOnSpline, also some SplineRoll cleanup

[glabute]

Purpose of this PR

CMCL-1591

New feature! Specifies LookAt data points for positions along a spline, so that a dolly camera looks at those places when it reaches specific positions on the spline.

New scene view tool to edit those points (works like SplineRoll tool):

Testing status

• Added an automated test
• Passed all automated tests
• Manually tested

Documentation status

• Updated CHANGELOG
• Updated README (if applicable)
• Commented all public classes, properties, and methods
• Updated user documentation

Technical risk

low

Comments to reviewers

The tool icon is a placeholder, @JulienDidierjean is taking care of providing new icons.
Possible improvements (to think about):

• Add option to specify a Transform to look at instead of a hardcoded point
• Prettier inspector (Data Points list is ugly - but this is provided by the spline team)
• Direct access to tool via button on inspector
• Interacting with tool auto-selects the appropriate list item in inspector
• For some reason, SaveDuringPlay does not capture modifications to the data points

@sebastienduverne Please review the user manual changes and figure out where to put links to the page.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 2.22222% with 176 lines in your changes are missing coverage. Please review.

Project coverage is 26.71%. Comparing base (7c09510) to head (2590cc7).

Files	Patch %	Lines
...tor/Editors/CinemachineLookAtDataOnSplineEditor.cs	0.00%	71 Missing ⚠️
...nity.cinemachine/Editor/Overlays/SplineRollTool.cs	0.00%	36 Missing ⚠️
...untime/Components/CinemachineLookAtDataOnSpline.cs	10.52%	34 Missing ⚠️
...hine/Editor/Editors/CinemachineSplineCartEditor.cs	0.00%	12 Missing ⚠️
...ine/Editor/Editors/CinemachineSplineDollyEditor.cs	0.00%	12 Missing ⚠️
...ity.cinemachine/Editor/Utility/InspectorUtility.cs	0.00%	9 Missing ⚠️
...hine/Editor/Editors/CinemachineSplineRollEditor.cs	0.00%	2 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
- Coverage   26.83%   26.71%   -0.12%     
==========================================
  Files         249      251       +2     
  Lines       28000    28138     +138     
==========================================
+ Hits         7513     7517       +4     
- Misses      20487    20621     +134     

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[glabute]

Opening a new PR, splitting this one into new features and fixed to old stuff