Revert "THRIFT-4982 Remove deprecated C# bindings from the code base"

Only compiler, test, lib and tutorial code.
diff --git a/test/ThriftTest.thrift b/test/ThriftTest.thrift
index ac49aee..bdc6d87 100644
--- a/test/ThriftTest.thrift
+++ b/test/ThriftTest.thrift
@@ -23,6 +23,7 @@
 
 namespace c_glib TTest
 namespace cpp thrift.test
+namespace csharp Thrift.Test
 namespace delphi Thrift.Test
 namespace go thrifttest
 namespace java thrift.test