Fix .NET SDK issue by specifying a namespace

The SDK was encountering errors with various operations (e.g., QUERY, RAWQUERY, SET, UPSERT) that interact directly with the database. This commit fixes the issue for SurrealDB V2.X, but it is not compatible with V1.X.
Fixes surrealdb#127:
Changes included:
-  Updated const strings for HTTP request headers from 'NS' to 'surrealdb-ns' and 'DB' to 'surrealdb-db'.

SurrealDb.Net/Internals/Constants/HttpConstants.cs

@@ -3,6 +3,6 @@
 internal static class HttpConstants
 {
     public const string ACCEPT_HEADER_NAME = "Accept";
-    public const string NS_HEADER_NAME = "NS";
-    public const string DB_HEADER_NAME = "DB";
+    public const string NS_HEADER_NAME = "surreal-ns";
+    public const string DB_HEADER_NAME = "surreal-db";
 }