Add FileProviderImageProvider and WebRootImageProvider

[ronaldbarendse]

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Description

This adds a FileProviderImageProvider and WebRootImageProvider, following the pattern mentioned in umbraco/Umbraco-CMS#12185 (comment). This is basically a simplified version of PR #207, but these IImageProviders are now included as additional, non-default implementations.

This would allow users to easily switch from the default PhysicalFileSystemProvider to the WebRootImageProvider by using:

services.AddImageSharp()
    .RemoveProvider<PhysicalFileSystemProvider>() // or .ClearProviders()
    .AddProvider<WebRootImageProvider>();

[ronaldbarendse]

Should this also do a null check?

Suggested change

	public FileProviderImageResolver(IFileInfo fileInfo) => this.fileInfo = fileInfo;
	public FileProviderImageResolver(IFileInfo fileInfo) => this.fileInfo = fileInfo ?? throw new ArgumentNullException(nameof(fileInfo));

[JimBobSquarePants]

I wouldn't bother. The default implementation is smart enough to only call this type when the IFileInfo is not null. If someone doesn't do this with their custom code then it's up to them.

[codecov]

Codecov Report

Merging #243 (9d34c2c) into main (249be0d) will decrease coverage by 0%.
The diff coverage is 0%.

@@         Coverage Diff         @@
##           main   #243   +/-   ##
===================================
- Coverage    85%    84%   -1%     
===================================
  Files        72     75    +3     
  Lines      2020   2036   +16     
  Branches    291    294    +3     
===================================
  Hits       1724   1724           
- Misses      212    228   +16     
  Partials     84     84           

Flag	Coverage Δ
unittests	84% <0%> (-1%)	⬇️

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Impacted Files	Coverage Δ
...geSharp.Web/Providers/FileProviderImageProvider.cs	0% <0%> (ø)
...c/ImageSharp.Web/Providers/WebRootImageProvider.cs	0% <0%> (ø)
...geSharp.Web/Resolvers/FileProviderImageResolver.cs	0% <0%> (ø)

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