Add fluent options builder

[Tarmil]

This PR introduces a new fluent syntax to define the options of the converter.

// New syntax:
let options =
    JsonFSharpOptions.Default()
        .WithUnionAdjacentTag()
        .WithUnionNamedFields()
        .WithUnwrapOption(false)
        .WithUnionTagName("type")
        .ToJsonSerializerOptions()

// Current syntax equivalent:
let options = JsonSerializerOptions()
JsonFSharpConverter(
    (JsonUnionEncoding.AdjacentTag
    ||| JsonUnionEncoding.NamedFields)
    &&& ~~~JsonUnionEncoding.UnwrapOption,
    unionTagName = "type")
|> options.Converters.Add

New API:

• Initializer methods:
  • .Default(), .NewtonsoftLike(), .ThothLike(), .FSharpLuLike() that create options with the corresponding JsonUnionEncoding.
• Fluent option methods:
  • A method .WithUnionFoo(?bool) for every enum case JsonUnionEncoding.Foo;
  • A method .WithFoo(value) for every optional argument foo of the JsonFSharpConverter constructor.
• Build methods:
  • .ToJsonSerializerOptions() creates a JsonSerializerOptions with an F# converter;
  • .AddToJsonSerializerOptions(options) adds the F# converter to an existing JsonSerializerOptions, useful to integrate with ASP.NET Core, Giraffe, or other libraries.

Advantages:

• More uniform syntax for the various options, instead of having some passed as enum and others as optional arguments.
• Better binary compatibility: adding an optional argument to the JsonFSharpConverter constructor is binary-breaking (see Congrats on 1.0, please avoid binary breaking changes 😊 #132), whereas adding a fluent method isn't.
• Allows creating a JsonSerializerOptions in a single expression, without having to create it and then mutate it. Especially useful when creating global options in a module.
• Easier to create a base of common options and then alter them slightly for overrides:

  let commonOptions =
      JsonFSharpOptions.Default()
          .WithUnionAdjacentTag()
          .WithUnionNamedFields()
  
  let options =
      commonOptions
          .WithOverrides(dict [
              typeof<MyCustomType>, commonOptions.WithUnionTagName("type")
              typeof<OtherCustomType>, commonOptions.WithUnionUnwrapSingleFieldCases()
          ])
          .ToJsonSerializerOptions()

Unresolved questions:

• The fluent syntax looks very C#-ish, even though it works perfectly fine in F#. Should we also add a more F#-ish syntax with module functions?

  let options =
      JsonFSharpOptions.Default()
      |> JsonFSharpOptions.unionAdjacentTag
      |> JsonFSharpOptions.unionNamedFields true
      |> JsonFSharpOptions.unwrapOption false
      |> JsonFSharpOptions.unionTagName "type"
      |> JsonFSharpOptions.toJsonSerializerOptions

• What about JsonFSharpConverterAttribute? Since it's an attribute, it can't be fluent. Perhaps it should have settable properties that do the same thing as corresponding fluent methods, eg:

  [<JsonFSharpConverter(UnionNamedFields = true, UnwrapOption = false)>]
  type Foo = // ...

[cmeeren]

• The fluent syntax looks very C#-ish, even though it works perfectly fine in F#. Should we also add a more F#-ish syntax with module functions?

I'm a bit late to the party, but for the record, I don't think the "F# alternative" adds any value. Fluent syntax works great in F#. Indeed, object programing (using classes etc.) works great and is even recommended in the official F# coding guidelines (not OOO stuff like inheritance, but using interfaces and classes).

[Tarmil]

@cmeeren Yes, I ended up not implementing it.

[cmeeren]

Just to be safe, can you verify that the old-style

options.Converters.Add(
  JsonFSharpConverter(
    unionEncoding = (
      JsonUnionEncoding.Default
      ||| JsonUnionEncoding.ExternalTag
      ||| JsonUnionEncoding.NamedFields
      ||| JsonUnionEncoding.UnwrapFieldlessTags
      ||| JsonUnionEncoding.UnwrapSingleFieldCases
      ||| JsonUnionEncoding.UnionFieldNamesFromTypes),
    unionFieldNamingPolicy = JsonNamingPolicy.CamelCase
  )
)

is equivalent to the new-style

options.Converters.Add(
  JsonFSharpOptions
    .Default()
    .WithUnionExternalTag()
    .WithUnionNamedFields()
    .WithUnionUnwrapFieldlessTags()
    .WithUnionUnwrapSingleFieldCases()
    .WithUnionFieldNamesFromTypes()
    .WithUnionFieldNamingPolicy(JsonNamingPolicy.CamelCase)
  |> JsonFSharpConverter
)

?

[Tarmil]

Yes, it's equivalent. Plus, this:

fsOptions.AddToJsonSerializerOptions(options)

is equivalent to:

JsonFSharpConverter(fsOptions)
|> options.Converters.Add