Add Memory3D<T> and Span3D<T>

[Ethereal77]

** Closes #723 **

This pull request adds four new types (Span3D<T> and Memory3D<T>, and their read-only counterparts) to allow working with 3D / volumetric data more confortably, as well as adding related extensions for arrays and spans to more easily interoperate with these new types and 3D indexing semantics.

• Added Span3D<T>, ReadOnlySpan3D<T>, Memory3D<T>, and ReadOnlyMemory3D<T>.
• Added AsSpan3D and AsMemory3D extension methods for T[,,] arrays.
• Added AsMemory3D extension methods for Memory<T> and ReadOnlyMemory<T>.
• Added AsSpan3D extension methods for Span<T> and ReadOnlySpan<T>.
• Added GetRow, GetColumn, and GetDepthColumn extension methods for T[,,] arrays, and as part of the new types, allowing efficient enumeration of 3D array rows, columns, and depth columns via RefEnumerable<T> or ReadOnlyRefEnumerable<T>.
• Introduced GetRowSpan and GetRowMemory methods to retrieve a Span<T> or Memory<T> over a specific row in a 3D array.

There are also new tests that covers every new type and functionality, mimicking as close as possible the tests already existing for the 2D methods/types already in the library.

PR Checklist

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• PR doesn't include merge commits (always rebase on top of our main, if needed)
• Tested code with current supported SDKs
• New component
  • Pull Request has been submitted to the documentation repository instructions. Link:
  • Added description of major feature to project description for NuGet package (4000 total character limit, so don't push entire description over that)
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes
• Every new API (including internal ones) has full XML docs
• Code follows all style conventions

Other information

Currently, the PR builds succesfully, and tests pass. There is some things I would like to discuss prior to consider this complete:

• Naming:
  • I have named the divisions of a volume along its depth dimension as "slices". I don't know if that is a good name. Some other names I've thought about were "layers", but that doesn't sound any better to me.
  • I have named a sequence of elements along the depth dimension a "depth column". Not very satisfied with that name either.
• While creating the new types, I've encountered some inconsisntencies in Span2D regarding some XML docs. I presume that would need a new issue/PR.
• I've tried to focus on just the wrapper span/memory types, and associated extensions to/from arrays/spans/memory. I've seen there is currently support for 2D spans in ParallelHelper. I presume this is out of scope, and matter for another PR too.
• Regarding documentation:
  • Should I create the documentation PR mentioned in the checklist now? Can it be done later when naming is agreed upon and the PR is accepted?
  • The same question regarding the "project description for NuGet package".