Allow overriding property and union case attributes in options

[Tarmil]

Fixes #181.

Add member WithOverrideMembers taking IDictionary<string, seq<Attribute>> to JsonFSharpOptions.

Example uses:

• Override attributes on record fields:

  type MyRecord = { x: int }
  
  let options =
      JsonFSharpOptions.Default()
          .WithOverrides(fun o -> dict [
              typeof<MyRecord>, o.WithOverrideMembers(dict [
                  "x", [JsonNameAttribute "y"]
              ])
          ])
          .ToJsonSerializerOptions()
  
  JsonSerializer.Serialize({ x = 1 }, o)
  // --> {"y":1}

• Override attributes on union cases:

  let options =
      JsonFSharpOptions.Default()
          .WithOverrides(fun o -> dict [
              typedefof<Result<_, _>>, o
                  .WithUnionInternalTag()
                  .WithUnionNamedFields()
                  .WithUnionTagName("isSuccess")
                  .WithOverrideMembers(dict [
                      nameof Ok, [
                          JsonNameAttribute true
                          JsonNameAttribute("value", Field = "ResultValue")
                      ]
                      nameof Error, [
                          JsonNameAttribute false
                          JsonNameAttribute("error", Field = "ErrorValue")
                      ]
                  ])
          ])
  
  JsonSerializer.Serialize(Ok 42, options)
  // --> {"isSuccess":true,"value":42}
  
  JsonSerializer.Serialize(Error "Internal error", options)
  // --> {"isSuccess":false,"error":"Internal error"}