Add the support for image data observable on ImageManager

[dreampiggy]

Add the support to observe the image data loaded success. This is used for user who want to have some extra setup logic for image data (not image bitmap object (UIImage).

However, due to the API semversion, we can not break current onSuccess API, which only receive the (UIImage, SDImageCacheType). Which will be changed in v2.0.0

So I just only add the imageData published property on manager.

See #104

[codecov]

Codecov Report

Merging #107 into master will increase coverage by 2.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
+ Coverage   72.78%   74.82%   +2.04%     
==========================================
  Files           9        9              
  Lines         823      878      +55     
==========================================
+ Hits          599      657      +58     
+ Misses        224      221       -3     

Flag	Coverage Δ
#ios	74.82% <100.00%> (+2.04%)	⬆️

Impacted Files	Coverage Δ
SDWebImageSwiftUI/Classes/ImageManager.swift	80.72% <100.00%> (+5.00%)	⬆️
SDWebImageSwiftUI/Classes/WebImage.swift	84.18% <0.00%> (+4.50%)	⬆️

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