Copy Azure stream to prevent read errors

[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

• Exposes ChunkedMemoryStream as a public type.
• Ensures entire Azure Blob stream is copied before processing.

[codecov]

Codecov Report

Merging #86 into master will increase coverage by 0.71%.
The diff coverage is 72.41%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   75.83%   76.55%   +0.71%     
==========================================
  Files          47       47              
  Lines        1316     1322       +6     
  Branches      228      230       +2     
==========================================
+ Hits          998     1012      +14     
+ Misses        254      242      -12     
- Partials       64       68       +4

Flag	Coverage Δ
#unittests	76.55% <72.41%> (+0.71%)	⬆️

Impacted Files	Coverage Δ
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	73.46% <100%> (ø)	⬆️
...s.Azure/Resolvers/AzureBlobStorageImageResolver.cs	90% <100%> (+10%)	⬆️
src/ImageSharp.Web/ChunkedMemoryStream.cs	75.48% <68.62%> (+4.94%)	⬆️
...s.Azure/Providers/AzureBlobStorageImageProvider.cs	76.47% <0%> (-5.89%)	⬇️

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