Replace Newtonsoft.Json with System.Text.Json so that we don't
have different libraries for JSON, because System.Text.Json
will be used by GRPC services.

Use System.Text.Json instead of Newtonsoft.Json in integration
tests.

Implement JSON converters for F# Discriminated Union types
because System.Text.Json can't serialize them out of the box.
Without converters, got the following error:
```
System.NotSupportedException : F# discriminated union serialization is not supported. Consider authoring a custom converter for the type.
The unsupported member type is located on type 'FsharpExchangeDotNetStandard.Currency'. Path: $.Market.BuyCurrency.
```