Expose the read write to FLAnimatedImage associate to the UIImage to allow advanced feature like placeholder

[dreampiggy]

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This merge request fixes / reffers to the following issues: #2217

Pull Request Description

In SD 4.3.0. We introduce a way to use associate object to bind FLAnimatedImage to UIImage. This allow user to "store" the FLAnimatedImage into our memory cache and fix some potential issue. At that time I mark this property readonly because I think user should avoid touching this in most cases.

However, some user also argue that they need to set placeholder of a FLAnimatedImage instance. Since FLAnimatedImage is not a UIImage subclass, they can not do this. So maybe we should just open this ability to allow write to that associate type. User now just set a FLAnimatedImage to UIImage placeholder and all things done.

This will not cause API-breaking because readwrite is superset of readonly. This is used for advanced feature and user should have the knowledge of what they are doing.

[codecov-io]

Codecov Report

Merging #2220 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2220   +/-   ##
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  Coverage   78.27%   78.27%           
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  Files          36       36           
  Lines        3664     3664           
  Branches      338      339    +1     
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  Hits         2868     2868           
  Misses        773      773           
  Partials       23       23

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[bpoplauschi]

I agree with this opening of the API