Add support for using a spring generator as a timing curve.

[jverkoey]

No description provided.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@69469ae). Click here to learn what that means.
The diff coverage is 84.74%.

@@            Coverage Diff             @@
##             develop      #91   +/-   ##
==========================================
  Coverage           ?   86.01%           
==========================================
  Files              ?       24           
  Lines              ?     2303           
  Branches           ?      136           
==========================================
  Hits               ?     1981           
  Misses             ?      309           
  Partials           ?       13

Impacted Files	Coverage Δ
src/private/CABasicAnimation+MotionAnimator.m	97.01% <100%> (ø)
tests/unit/SpringTimingCurveTests.swift	78.57% <78.57%> (ø)

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