[release/6.0-preview7] Enable the generic math feature

[tannergooding]

Backport of #55678. Issue dotnet/designs#205

Issue Summary

C++/CLI fails to compile any code that utilizes a type implementing an interface with a static abstract member. Since generic math involves implementing interfaces with static abstract members on the primitive types, this impacted nearly every C++/CLI compilation.

This works around the issue by creating a System.Runtime.Experimental package that will override the System.Runtime.dll that ships in box. This experimental reference assembly exposes the generic math methods and interfaces allowing projects to individually opt-in and thus avoids the issue of tooling, such as C++/CLI, encountering the new IL by default.

Customer Impact

Customers will not be able to utilize the generic math preview feature and early feedback of the feature will not be as feasible.

Testing

Manual validation that .NET works with C++/CLI by default and that the functionality is usable from a C# app explicitly referencing the experimental package was done.

Regression?

No. These are net new preview types.

Risk

Low. Product has already undergone decent testing and perf validation. It was pulled out at last minute due to an unforeseen incompatibility with C++/CLI. The C++/CLI team is aware of the problem and will be looking into it further to determine the correct fix.

This functionality exposes a way for the feature to still be useable, via explicit opt-in, without impacting any existing tools that are otherwise not currently compatible.

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[tannergooding]

CC. @jeffhandley, @ericstj, @ViktorHofer, @terrajobst, @davidwrighton, @MadsTorgersen

This is the backport to .NET 6 Preview 7. It missed the cutoff due to a WASM AOT issue (#55767) that had cropped up when it tried to execute the experimental test package.

[davidwrighton]

The change looks good

[tannergooding]

runtime (Libraries Test Run release mono Linux x64 Debug) was a timeout in System.Net.Quic.Functional.Tests and appears unrelated (#55642). CI, for whatever reason, marked the main runtime job as passing and so there doesn't appear to be a way to rerun this singular job.

[tannergooding]

This was approved by Steve Carroll over tactics e-mail. What's required to get it merged now (branch is protected and restricted to authorized users)?

[danmoseley]

Merged. I'm not quite sure how it works, but I can merge into this branch if it's green, but not otherwise. In that case I ping on the infra channel.

[danmoseley]

@tannergooding it would be good to keep an eye on codeflow and when a P7 installer comes out with this in, do another quick check.

[tannergooding]

Thanks @danmoseley and will do