public Optional<String> getIdentitySource() {
        return Optional.ofNullable(identitySource);
    }
Gets the comma-separated list of mapping expressions of the request
parameters as the identity source.

<p>This property is only applicable for the authorizer of the
"request" type only.

@return Returns the optional identity source string.
smithy-aws-apigateway-traits/src/main/java/software/amazon/smithy/aws/apigateway/traits/AuthorizerDefinition.java