Allow default values for non-builtIn endpoint params (#6084)

Author: davidh44

.changes/next-release/bugfix-AWSSDKforJavav2-175fc73.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "AWS SDK for Java v2",
+    "contributor": "",
+    "description": "Allow default values for non-builtIn endpoint params"
+}

codegen/src/main/resources/software/amazon/awssdk/codegen/rules/Parameter.java.resource

@@ -22,9 +22,6 @@ public final class Parameter implements ToParameterReference {
     private final boolean required;
 
     public Parameter(Builder builder) {
-        if (builder.defaultValue != null && builder.builtIn == null) {
-            throw new RuntimeException("Cannot set a default value for non-builtin parameters");
-        }
         if (builder.defaultValue != null && !builder.required) {
             throw new RuntimeException("When a default value is set, the field must also be marked as required");
         }