Expose Convolution Api

[JimBobSquarePants]

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

See the roadmap discussion at https://github.com/SixLabors/ImageSharp/discussions/2533

Adds a general convolution API that accepts a DenseMatrix<float> to describe the convolution kernel.

All this stuff has been the plumbing for various processors over the years and should be exposed publicly now it's solid.