fix: graal typehint for SingleValueHeaders (#1549)

Native executable was failing in AWS Lambda with:

```
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.amazonaws.serverless.proxy.model.SingleValueHeaders` (no Creators, like default constructor, exist): no default constructor found
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.amazonaws.serverless.proxy.model.SingleValueHeaders` (no Creators, like default constructor, exist): no default constructor found
```

function-aws-api-proxy/src/main/java/io/micronaut/function/aws/proxy/MicronautLambdaContainerHandler.java

@@ -115,6 +115,7 @@
                 ContainerConfig.class,
                 ErrorModel.class,
                 Headers.class,
+                SingleValueHeaders.class,
                 MultiValuedTreeMap.class,
                 AwsProxySecurityContext.class
         }