Investigate the use of discriminated union in lieu of ActionResult<T>

[pranavkm]

The feature is still in proposal but if it lands, we should investigate replacing ActionResult<T> with a compiler feature.

Current spec: https://github.com/dotnet/csharplang/blob/master/proposals/discriminated-unions.md

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[pranavkm]

Closing this. The feature isn't planned for C# 10. We can file a new issue at a later date.