Add GrayscaleWebProcessor

[stefannikolei]

Prerequisites

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

Description

Adds an Grayscale processor as in #310

[codecov[bot]]

Codecov Report

Merging #313 (75f32e2) into main (135c8b1) will decrease coverage by 1%. The diff coverage is 75%.

@@         Coverage Diff         @@
##           main   #313   +/-   ##
===================================
- Coverage    85%    85%   -1%     
===================================
  Files        77     78    +1     
  Lines      2195   2206   +11     
  Branches    308    309    +1     
===================================
+ Hits       1876   1884    +8     
- Misses      231    233    +2     
- Partials     88     89    +1     

Flag	Coverage Δ
unittests	85% <75%> (-1%)	:arrow_down:

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Impacted Files	Coverage Δ
...ImageSharp.Web/Processors/GrayscaleWebProcessor.cs	70% <70%> (ø)
...DependencyInjection/ServiceCollectionExtensions.cs	100% <100%> (ø)

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[JimBobSquarePants]

Thanks, but I don't want to add this.

If we add everything people ask for then we vastly increase the attack surface for DDOS. The library is extensible and they have all the tools to add their own processors.