fix: add V202012 to SpecVersionDetector And JsonMetaSchema (#586)

Author: drunkcattt

src/main/java/com/networknt/schema/JsonMetaSchema.java

@@ -195,6 +195,10 @@ public static JsonMetaSchema getV201909() {
         return new Version201909().getInstance();
     }
 
+    public static JsonMetaSchema getV202012() {
+        return new Version202012().getInstance();
+    }
+
     /**
      * Builder without keywords or formats.
      * <p>

src/main/java/com/networknt/schema/SpecVersionDetector.java

@@ -51,6 +51,8 @@ else if (schemaUri.equals(JsonMetaSchema.getV7().getUri()))
             return SpecVersion.VersionFlag.V7;
         else if (schemaUri.equals(JsonMetaSchema.getV201909().getUri()))
             return SpecVersion.VersionFlag.V201909;
+        else if (schemaUri.equals(JsonMetaSchema.getV202012().getUri()))
+            return SpecVersion.VersionFlag.V202012;
         else
             throw new JsonSchemaException("Unrecognizable schema");
     }