feat: add StringEnum<TEnum>

[JamieMagee]

This PR adds the StringEnum<TEnum>, similar to Octokit's version¹. See Octokit documentation for more explanation and reasoning behind this approach²

TODO:

• Handle IEnumerable<StringEnum<TEnum>> with a custom JsonConvertorFactory³
• Refactor ToEnumString and ToEnum helper methods to extension methods (or somewhere else)
• StringEnumConverter (or StringEnumConverterFactory) tests

Closes #90

Footnotes

1. https://github.com/octokit/octokit.net/blob/5fe3ea82f03b7fe64bcf8145f3b127fa2c585665/Octokit/Models/Response/StringEnum.cs#L11 ↩

2. https://github.com/octokit/octokit.net/blob/main/docs/working-with-enums.md ↩

3. https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to?pivots=dotnet-6-0#sample-factory-pattern-converter ↩

[JamieMagee]

@martincostello I think this is ready for review

[JamieMagee]

Thanks for the review @martincostello! There are a couple of model updates that I'd like to merge and get into a patch release, before merging this and cutting a major release.

[JamieMagee]

I pushed an update, but it looks like the PR isn't receiving updates 😢

[kfcampbell]

Thanks for all your work here!

[JamieMagee]

@martincostello I just cut the v2.0.0 release. Can you upgrade and let me know how it goes?

[martincostello]

Sure - I'll poke dependabot on my app and see what happens 😄

[martincostello]

I just had to make a few minor adjustments to fix the compilation and maintain the behaviour: martincostello/costellobot@3e552ca

[JamieMagee]

Those changes look like what I would expect.

Thanks for all your help with this change!