CMCL-0000: Add Easing option to CinemachineSplineRoll

[glabute]

Purpose of this PR

Solves the problem raised by a client that there is no way to smoothly roll the camera on a spline.
https://discussions.unity.com/t/replicating-cm2s-smoothpath-in-cm3s-splines/952265/20

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

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 16 lines in your changes missing coverage. Please review.

Project coverage is 26.95%. Comparing base (e5a39a9) to head (eee8004).

Files	Patch %	Lines
...achine/Runtime/Behaviours/CinemachineSplineRoll.cs	0.00%	12 Missing ⚠️
...hine/Editor/Editors/CinemachineSplineRollEditor.cs	0.00%	3 Missing ⚠️
...emachine/Runtime/Core/SplineContainerExtensions.cs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@                              Coverage Diff                               @@
##           dev/add-CinemachineVirtualCameraBaseEditor    #1016      +/-   ##
==============================================================================
- Coverage                                       26.96%   26.95%   -0.02%     
==============================================================================
  Files                                             254      254              
  Lines                                           28425    28439      +14     
==============================================================================
  Hits                                             7666     7666              
- Misses                                          20759    20773      +14     

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