CMCL-1606: TargetPositionCache needs to be public

[glabute]

Purpose of this PR

Custom Cinemachine classes were prevented from supporting CInemachine's Timeline cached scrubbing feature because the necessary API was internal.

Solution was to make public the following two static methods: TargetPositionCache.GetTargetPosition() and TargetPositionCache.GetTargetRotation()

Testing status

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

Documentation status

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

Technical risk

0

Comments to reviewers

Just made 2 static methods public. There is a repro project in the bug report that can be used to test.