Fix DecoderOptions handler.

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

The current OnBeforeLoadAsync delegate is useless as DecoderOptions properties are init-only. This PR includes an emergency breaking change which updates the method to return options themselves.

[codecov]

Codecov Report

Merging #323 (6071b4e) into main (4fdc000) will increase coverage by 0%.
The diff coverage is 100%.

@@         Coverage Diff         @@
##           main   #323   +/-   ##
===================================
  Coverage    85%    85%           
===================================
  Files        83     83           
  Lines      2371   2371           
  Branches    346    347    +1     
===================================
+ Hits       2024   2026    +2     
+ Misses      244    242    -2     
  Partials    103    103           

Flag	Coverage Δ
unittests	85% <100%> (+<1%)	⬆️

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Impacted Files	Coverage Δ
.../ImageSharp.Web/Middleware/ImageSharpMiddleware.cs	83% <100%> (ø)
...harp.Web/Middleware/ImageSharpMiddlewareOptions.cs	93% <100%> (ø)

... and 1 file with indirect coverage changes

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