Expose Pixel Operations and Configuration

[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 the various IPixel operations as bulk operations. Individual overriding classes remain internal.
• Normalize Configuration usage and expose GetConfiguration() extension for ImageFrame
• Normalize MemoryAllocator usage and expose only via Configuration.

Fix #1092

[codecov]

Codecov Report

Merging #1095 into master will increase coverage by <.01%.
The diff coverage is 97.71%.

@@            Coverage Diff             @@
##           master    #1095      +/-   ##
==========================================
+ Coverage   81.49%   81.49%   +<.01%     
==========================================
  Files         704      704              
  Lines       29381    29389       +8     
  Branches     3289     3290       +1     
==========================================
+ Hits        23944    23951       +7     
  Misses       4745     4745              
- Partials      692      693       +1

Flag	Coverage Δ
#unittests	81.49% <97.71%> (ø)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Configuration.cs	100% <ø> (ø)	⬆️
src/ImageSharp/Image{TPixel}.cs	90% <100%> (ø)	⬆️
...rocessors/Quantization/WuFrameQuantizer{TPixel}.cs	91.79% <100%> (ø)	⬆️
...ssors/Overlays/BackgroundColorProcessor{TPixel}.cs	100% <100%> (ø)	⬆️
...sors/Quantization/PaletteFrameQuantizer{TPixel}.cs	100% <100%> (ø)	⬆️
...ntations/Generated/L8.PixelOperations.Generated.cs	100% <100%> (ø)	⬆️
...ations/Generated/La16.PixelOperations.Generated.cs	98.16% <100%> (ø)	⬆️
...ats/PixelImplementations/Rgba32.PixelOperations.cs	100% <100%> (ø)	⬆️
...tions/Generated/Rgb48.PixelOperations.Generated.cs	100% <100%> (ø)	⬆️
...PixelImplementations/RgbaVector.PixelOperations.cs	100% <100%> (ø)	⬆️
... and 25 more

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[antonfirsov]

Just a naming nit, otherwise LGTM.