Use const where appropriate

The value of a const field is computed at compile time and stored in the metadata, which improves run-time performance when it is compared to a static readonly field.

src/Sonarr.Http/ClientSchema/SchemaBuilder.cs

@@ -13,7 +13,7 @@ namespace Sonarr.Http.ClientSchema
 {
     public static class SchemaBuilder
     {
-        private static readonly string PRIVATE_VALUE = "********";
+        private const string PRIVATE_VALUE = "********";
         private static Dictionary<Type, FieldMapping[]> _mappings = new Dictionary<Type, FieldMapping[]>();
 
         public static List<Field> ToSchema(object model)