[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/namespace_test/NamespaceA/NamespaceB/EnumInNestedNS.cs

@@ -5,6 +5,7 @@
 namespace NamespaceA.NamespaceB
 {
 
+[Newtonsoft.Json.JsonConverter(typeof(Newtonsoft.Json.Converters.StringEnumConverter))]
 public enum EnumInNestedNS : sbyte
 {
   A = 0,

tests/namespace_test/NamespaceA/NamespaceB/StructInNestedNS.cs

@@ -47,7 +47,9 @@ public struct StructInNestedNS : IFlatbufferObject
 
 public class StructInNestedNST
 {
+  [Newtonsoft.Json.JsonProperty("a")]
   public int A { get; set; }
+  [Newtonsoft.Json.JsonProperty("b")]
   public int B { get; set; }
 
   public StructInNestedNST() {

tests/namespace_test/NamespaceA/NamespaceB/TableInNestedNS.cs

@@ -53,6 +53,7 @@ public struct TableInNestedNS : IFlatbufferObject
 
 public class TableInNestedNST
 {
+  [Newtonsoft.Json.JsonProperty("foo")]
   public int Foo { get; set; }
 
   public TableInNestedNST() {