Add support for JsonSnakeNamingPolicy

[hez2010]

• Add JsonSnakeNamingPolicy support
• Add tests for JsonSnakeNamingPolicy

Fixed #39564

[Gnbrkm41]

Shouldn't this go through the API review process though, no? https://github.com/dotnet/corefx/blob/master/Documentation/project-docs/api-review-process.md

[hez2010]

@Gnbrkm41 It had been added to the 5.0 milestone so I think that I can implement it in advance :)
And it's really useful in json serialization/deserialization, if there's no snake case in System.Text.Json, many people who migrate from Newtonsoft.Json to System.Text.Json would face breaking changes and they will have to implement it by themselves.

[Gnbrkm41]

Being added to milestones doesn't mean it's approved; every single thing about the public APIs should be thoroughly discussed, marked as ready for review when the area owner thinks it's solid enough, reviewed by the review team then approved in order to actually be implemented.

The Pull Request section of the API review process document says:

Pull requests against corefx shouldn't be submitted before getting approval. Also, we don't want to get work in progress (WIP). The reason being that we want to reduce the number pending PRs so that we can focus on the work the community expects we take action on.

[danmoseley]

I'm going to close for reason mentioned above. Let's continue discussion on the issue. Thanks.