Simplify middleware workflow.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Simplifies the middleware workflow and removes the requirement for stream copying in individual providers.
Adds tests to ensure we handle cached and non-cached scenarios.

[codecov]

Codecov Report

Merging #110 into master will increase coverage by 0.86%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   82.03%   82.89%   +0.86%     
==========================================
  Files          44       44              
  Lines        1091     1076      -15     
  Branches      160      158       -2     
==========================================
- Hits          895      892       -3     
+ Misses        145      136       -9     
+ Partials       51       48       -3     

Flag	Coverage Δ
#unittests	82.89% <84.61%> (+0.86%)	⬆️

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

Impacted Files	Coverage Δ
...ndencyInjection/ImageSharpCoreBuilderExtensions.cs	75.86% <ø> (-2.27%)	⬇️
...DependencyInjection/ServiceCollectionExtensions.cs	86.36% <ø> (-0.60%)	⬇️
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	78.00% <83.60%> (+6.57%)	⬆️
...s.Azure/Providers/AzureBlobStorageImageProvider.cs	77.27% <100.00%> (-1.46%)	⬇️
...s.Azure/Resolvers/AzureBlobStorageImageResolver.cs	100.00% <100.00%> (+7.69%)	⬆️
...eSharp.Web/Providers/PhysicalFileSystemProvider.cs	85.00% <0.00%> (+5.00%)	⬆️
...arp.Web/Caching/PhysicalFileSystemCacheResolver.cs	100.00% <0.00%> (+16.66%)	⬆️

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[JimBobSquarePants]

I'm gonna go rogue here and merge this. I've got limited availability to work on the library so want to press on while I can.