Feature AnimatedImage placeholder view builder

[dreampiggy]

This PR supports the @ViewBuilder API to create placeholder on AnimatedImage.

The channlege is that AnimatedImage use UIViewRepresentable, which the body func is override by SwiftUI internel logic. So we can not use the same design as WebImage (which return different Image based on loading status)

So, instead, we use the HostingView, to inject the View Hierarchy. The placeholder is above the original SDAnimatedImageView, and controls its isHidden property during loading.

Note the placeholder and indicator can be used at the same time, we need to considerate this case and adjust the View Hierarchy level

[codecov[bot]]

Codecov Report

Merging #94 into master will increase coverage by 2.03%. The diff coverage is 85.96%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   73.49%   75.53%   +2.03%     
==========================================
  Files           9        9              
  Lines         732      748      +16     
==========================================
+ Hits          538      565      +27     
+ Misses        194      183      -11     

Flag	Coverage Δ
#ios	75.53% <85.96%> (+2.03%)	:arrow_up:

Impacted Files	Coverage Δ
SDWebImageSwiftUI/Classes/ImageViewWrapper.swift	82.69% <77.77%> (-1.03%)	:arrow_down:
SDWebImageSwiftUI/Classes/WebImage.swift	84.51% <80.00%> (+4.51%)	:arrow_up:
SDWebImageSwiftUI/Classes/ImageManager.swift	75.71% <83.33%> (+4.44%)	:arrow_up:
SDWebImageSwiftUI/Classes/AnimatedImage.swift	73.89% <90.32%> (+1.29%)	:arrow_up:

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