CMCL-1603: CM2 backport: Added Padding option to CinemachineConfiner2D

[glabute]

Purpose of this PR

Backport of CMCL-1603 to CM2. See https://github.com/Unity-Technologies/com.unity.cinemachine/pull/1018

Testing status

• [ ] Added an automated test
• [x] Passed all automated tests
• [x] Manually tested

Documentation status

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

Technical risk

low

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 43.75000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 26.54%. Comparing base (99f33a2) to head (4d87d56).

Files	Patch %	Lines
...hine/Editor/Editors/CinemachineConfiner2DEditor.cs	0.00%	8 Missing :warning:
...achine/Runtime/Behaviours/CinemachineConfiner2D.cs	75.00%	1 Missing :warning:

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Additional details and impacted files

@@              Coverage Diff              @@
##           release/2.10    #1019   +/-   ##
=============================================
  Coverage         26.54%   26.54%           
=============================================
  Files               126      126           
  Lines             18926    18933    +7     
=============================================
+ Hits               5023     5025    +2     
- Misses            13903    13908    +5     

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