Promote System.Text.Json as default JSON serializer

[esbenbjerre]

Description

This PR removes the Newtonsoft JSON serializer and promotes System.Text.Json as the default. I think System.Text.Json is the better choice these days. Among other things the default serializer

• Uses camel-case
• Serializes Option as value or null
• Supports ignoring Option or null values using the [<JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingDefault)>] attribute

Any other specific behaviour should IMO be implemented by users, using attributes (e.g., [<JsonPropertyName("someName")>], [<JsonConverter(typeof<SomeCustomJsonConverter>)>]), as there's no sensible defaults for most user-defined types.

How to test

Run tests and try to serialize/deserialize a JSON payload in a HttpHandler.

Related issues

#481
#560
#451

[esbenbjerre]

@dustinmoris

[TheAngryByrd]

Why are you removing Newtonsoft entirely? I can see maybe swapping the default but not outright removal.

[esbenbjerre]

It was my impression from discussion in #451 that it would be preferable to have it in a separate NuGet package and I agree with that sentiment. No reason to include the dependency by default if it's not going to be used. I also think System.Text.Json have become much better for F# with .NET 6/7 updates.

[esbenbjerre]

I also think this switch would get us closer to finally having a tryBindJson function. I'll start working on that when/if this PR is approved.

[dbrattli]

Great work. Thanks for doing this!!

[64J0]

I'll re-review this PR during this week. Thanks for the updates @esbenbjerre!

[TheAngryByrd]

Thanks for the work on this!

---

@64J0 I want to mention I think this should be considered as a major version bump. Swapping JSON serialization can be a drastic change and shouldn't be taken lightly. A major bump should be a big signal that something has changed. It should also be very loud and clear in release notes with migration links to STJ or how to put back the Newtonsoft serializer.

[64J0]

Thanks for the heads-up @TheAngryByrd! Since this PR is already open for a while, and we have many approvals, I'm going to merge it right now. Thanks for the contribution @esbenbjerre, great job!