add Json/TypeConverters

[zyofeng]

I have added implementation for Json/TypeConverters which can be used for deserialization/model binding.

[davetransom]

Thanks @zyofeng, and sorry for the delay in responding to you.

I find it a bit hard to see the project diff with all the indentation changes.

Compatibility is important. As is having no external dependencies (it's a small library). You've removed targeting net40, but it looks like there needs to be work on this to add conditional references between net40 and netcoreapp3.1, as well as tests that prove the conversion behaves.

Do you want to make some changes to the PR?

Here's what this PR needs:

• [ ] No unnecessary whitespace changes to the .csproj file.
• [ ] Add net40 back to <TargetFrameworks/>.
• [ ] Use conditional compilation to target netcoreapp3.1 for System.Text.Json and its JsonConverter (since System.Text.Json is built-in to 3.1 i.e. FrameworkReference).
• [ ] Remove the external reference for System.Text.Json when targeting netstandard2.0 as it's a ProjectReference (external dependency). netstandard2.0 and net40 won't get the JsonConverter, only the TypeConverter.
• [ ] ShortGuidJsonConverter has a redundant clause: reader.GetString() ?? new ShortGuid(reader.GetString()). This should probably use an explicit cast (for clarity) e.g. (ShortGuid)reader.GetString() which will return ShortGuid.Empty for null or empty strings.
• [ ] ShortGuidTypeConverter should also allow for Guid conversion.
• [ ] Add a set of tests for each converter to prove both work as expected.

If you're like me to take it over, this is the list I'll be working through.

[zyofeng]

Feel free to take over, although net40 has long gone EOL