[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.

docs/source/CsharpUsage.md

@@ -151,4 +151,25 @@ To use:
     fbb.Finish(Monster.Pack(fbb, monsterobj).Value);
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+### Json Serialization
+
+An additional feature of the object API is the ability to allow you to
+serialize & deserialize a JSON text. 
+To use Json Serialization, add `--gen-json-serializer` option to `flatc` and
+add `Newtonsoft.Json` nuget package to csproj.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cs}
+    // Deserialize MonsterT from json
+    string jsonText = File.ReadAllText(@"Resources/monsterdata_test.json");
+    MonsterT mon = MonsterT.DeserializeFromJson(jsonText);
+
+    // Serialize MonsterT to json
+    string jsonText2 = mon.SerializeToJson();
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+* Limitation
+  * `hash` attribute currentry not supported.
+* NuGet package Dependency
+  * [Newtonsoft.Json](https://github.com/JamesNK/Newtonsoft.Json)
+
 <br>