CDK如何在使用from_role_arn从现有角色创建角色时添加假设角色

from aws_cdk import core, aws_iam

class MyStack(core.Stack):
    def __init__(self, scope: core.Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)

class MyStack(core.Stack):
    def __init__(self, scope: core.Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)
        
        # 创建一个现有角色
        existing_role_arn = '<现有角色的ARN>'
        existing_role = aws_iam.Role.from_role_arn(
            self, 'ExistingRole', role_arn=existing_role_arn
        )
        
        # 创建一个新的角色，并设置假设角色
        new_role = aws_iam.Role(
            self, 'NewRole',
            assumed_by=aws_iam.FederatedPrincipal(
                federated='cognito-identity.amazonaws.com',
                conditions={
                    'StringEquals': {
                        'cognito-identity.amazonaws.com:aud': '<Identity Pool ID>'
                    },
                    'ForAnyValue:StringLike': {
                        'cognito-identity.amazonaws.com:amr': 'authenticated'
                    }
                },
                assume_role_action='sts:AssumeRoleWithWebIdentity'
            )
        )
        
        # 添加假设角色到现有角色
        new_role.add_to_policy(
            aws_iam.PolicyStatement(
                actions=['sts:AssumeRole'],
                resources=[existing_role.role_arn]
            )
        )