[C#] support Json Serialization (#5752)

* support json serialization

* fix invalid json format.
* string must be written with double quotes.
* remove commma after the last object member.

* fix indent

* Revert "fix invalid json format."

This reverts commit d6820ed50c.

* quated string value.

* add cs-gen-json-serializer flag.

* fix preprocessor indent

* ENABLE_JSON_SERIALIZATION -> ENABLE_JSON_SERIALIZATION_TEST

* share TestBuffer method

* remove ENABLE_JSON_SERIALIZATION

* remove duplicated test data

* [windows] add nuget restore and copy test data.

* [docker mono] share msbuild settings with windows. add nuget restore and copy test data.

* add some note for json api.

tests/MyGame/Example/Referrable.cs

@@ -78,6 +78,8 @@ public struct Referrable : IFlatbufferObject
 
 public class ReferrableT
 {
+  [Newtonsoft.Json.JsonProperty("id")]
+  [Newtonsoft.Json.JsonIgnore()]
   public ulong Id { get; set; }
 
   public ReferrableT() {