Core: use FSharp.SystemTextJson library

Use FSharp.SystemTextJson library so that all F# types,
including discriminated unions, can be serialized using
System.Text.Json.

Without it, tests would fail with 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.
```

And after adding converters for `Currency` and `Side` types,
tests would fail at [1], indicating that something is wrong
with serialization.

[1] https://github.com/nblockchain/FX/blob/33ce6fa6d133bf3a685053798cad6a41712514e7/src/FX.Tests/RedisIntegrationTests.cs#L62

Directory.Packages.props

@@ -3,6 +3,7 @@
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
   </PropertyGroup>
   <ItemGroup>
+    <PackageVersion Include="FSharp.SystemTextJson" Version="1.2.42" />
     <PackageVersion Include="StackExchange.Redis" Version="2.0.513" />
     <PackageVersion Include="NUnit" Version="3.9.0" />
     <PackageVersion Include="NUnit3TestAdapter" Version="3.9.0" />

src/FX.Core/FX.Core.fsproj

@@ -12,6 +12,7 @@
     <Compile Include="Exchange.fs" />
   </ItemGroup>
   <ItemGroup>
+    <PackageReference Include="FSharp.SystemTextJson" />
     <PackageReference Include="StackExchange.Redis" />
     <PackageReference Include="System.Text.Json" />
   </ItemGroup>

src/FX.Core/RedisStorageLayer.fs

@@ -6,11 +6,12 @@ open System
 open FsharpExchangeDotNetStandard
 
 open System.Text.Json
+open System.Text.Json.Serialization
 open StackExchange.Redis
 
 [<AutoOpen>]
 module Serialization =
-    let serializationOptions = JsonSerializerOptions.Default
+    let serializationOptions = JsonFSharpOptions.Default().ToJsonSerializerOptions()
 
 type OrderQuery =
     {