Supports the user to customize the default disk cache directory, which can be used to share cache for App && App Extension

[dreampiggy]

New Pull Request Checklist

• I have read and understood the CONTRIBUTING guide

• I have read the Documentation

• I have searched for a similar pull request in the project and found none

• I have updated this branch with the latest master to avoid conflicts (via merge from master or rebase)

• I have added the required tests to prove the fix/feature I am adding

• I have updated the documentation (if necessary)

• I have run the tests and they pass

• I have run the lint and it passes (pod lib lint)

This merge request fixes / refers to the following issues: close #3050

Pull Request Description

This PR solve the feature request from #3050.

If you want to change the shared image cache's path, use like this:

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification {
    // Insert code here to initialize your application
    SDImageCache.defaultImageCacheDirectory = [NSFileManager.defaultManager containerURLForSecurityApplicationGroupIdentifier:@"my.app.group.id"];
}

[codecov]

Codecov Report

Merging #3066 into master will decrease coverage by 0.43%.
The diff coverage is 47.36%.

@@            Coverage Diff             @@
##           master    #3066      +/-   ##
==========================================
- Coverage   83.57%   83.14%   -0.44%     
==========================================
  Files          69       70       +1     
  Lines        7605     7617      +12     
==========================================
- Hits         6356     6333      -23     
- Misses       1249     1284      +35     

Flag	Coverage Δ
#ios	83.22% <50.00%> (-0.33%)	⬇️
#macos	83.22% <34.73%> (-0.24%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
SDWebImage/Core/NSData+ImageContentType.m	79.09% <ø> (-0.19%)	⬇️
SDWebImage/Core/SDImageAWebPCoder.m	0.00% <0.00%> (ø)
SDWebImage/Core/SDAnimatedImageRep.m	80.51% <20.00%> (-9.04%)	⬇️
SDWebImage/Core/SDImageHEICCoder.m	81.13% <50.00%> (+1.33%)	⬆️
SDWebImage/Core/SDImageIOAnimatedCoder.m	85.74% <91.66%> (+0.35%)	⬆️
SDWebImage/Core/SDImageCache.m	78.12% <100.00%> (+0.32%)	⬆️
SDWebImage/Core/SDImageIOCoder.m	88.70% <100.00%> (-0.25%)	⬇️
...DWebImage/Private/SDImageIOAnimatedCoderInternal.h	100.00% <100.00%> (ø)
... and 1 more

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