cln: add JsonConverter for handling enum

[joemphilips]

Unlike Newtonsoft.Json, System.Text.Json does not
respect EnumMemberAttribute. (see: dotnet/runtime#31081)
This caused an issue when sending request to c-lightning,
when you use System.Text.Json for serialization lib, and there
is a enum value in the request field, the field will be serialized into
an int, instead of a string. Thus causing RPC error.

This commit fixes it by using JsonStringEnumConverterEx
(which is taken from an issue comment of issue above).

[knocte]

I see you're trying to find the packages dir where nuget packages get downloaded. I recently had trouble with this because apparently M$ has changed behaviour; to go back to the old behaviour of creating a packages subfolder in the repo itself when restoring, you can create a NuGet.config file with this content:

<configuration>
    <config>
        <add key="globalPackagesFolder" value=".\packages\" />
    </config>
</configuration>

[joemphilips]

@knocte Thanks, but it seems this is a different issue.

It seems that fsdocs fails to find a path to 3rd party dll, (in this case NBitcoin)
ref: fsprojects/FSharp.Formatting#616

I don't know why but it works in local, even when I remove the nuget cache with dotnet nuget locals global-packages -c

I should probably change the publish workflow to more manual way, as I have to do it anyway for #224

[joemphilips]

It seems that fsdocs fails to find a path to 3rd party dll, (in this case NBitcoin)
ref: fsprojects/FSharp.Formatting#616
I don't know why but it works in local, even when I remove the nuget cache with dotnet nuget locals global-packages -c

@knocte hmm, it works fine when I use an old version of .NET SDK. Maybe the root cause is same with yours.
Anyway I should report this to fsdocs repo.