Cognito-idp: missing readable attributes in JWT payload

[ferruvich]

While testing the JWT payload verification part in my app, i've noticed that moto ignores the pool client readable attributes in JWT payload.
Here's a working example:

User Pool And App Client creation

cognito_client = boto3.client('cognito-idp')

create_response = cognito_client.create_user_pool(
    PoolName='user_pool'
)
client_id = cognito_client.create_user_pool_client(
    UserPoolId=create_response['UserPool']['Id'],
    ClientName='mock_pool_client',
    ReadAttributes=['custom:readable_attribute']
)['UserPoolClient']['ClientId']

User Creation

user = cognito_client.admin_create_user(
    UserPoolId=create_response['UserPool']['Id'],
    Username='mock@user.com',
    UserAttributes=[{
            'Name': 'custom:readable_attribute',
            'Value': 'readable_value'
    }],
    TemporaryPassword='temporary_password'
)['User']

User Confirmation and Token Retrieving

init_auth_response = cognito_client.admin_initiate_auth(
    UserPoolId=user_pool_id,
    ClientId=client_id,
    AuthFlow='ADMIN_NO_SRP_AUTH',
    AuthParameters={
        'USERNAME': user['Username'],
        'PASSWORD': 'temporary_password'
    },
)

auth_response = cognito_client.respond_to_auth_challenge(
    ClientId=client_id,
    Session=init_auth_response['Session'],
    ChallengeName=init_auth_response['ChallengeName'],
    ChallengeResponses={
        'USERNAME': user['Username'],
        'NEW_PASSWORD': 'definitive_password'
    }
)

In this way, we have the user's access token in auth_response['AuthenticationResult']['AccessToken'].

The token payload should be like this:

{
    "iss": "https://cognito-idp.eu-central-1.amazonaws.com/5f74a2d1-1965-4010-b5cd-a889e71abfab",
    "sub": "3e61257c-a209-4cbe-b408-da69c00f4352",
    "aud": "ecbe867f-9fac-42ee-8eea-e05e95157896",
    "token_use": "id",
    "auth_time": 1533646833,
    "exp": 1533650433,
    "custom:readable_attribute": "readable_value"
}

Instead, the payload appears to be like this:

{
    "iss": "https://cognito-idp.eu-central-1.amazonaws.com/5f74a2d1-1965-4010-b5cd-a889e71abfab",
    "sub": "3e61257c-a209-4cbe-b408-da69c00f4352",
    "aud": "ecbe867f-9fac-42ee-8eea-e05e95157896",
    "token_use": "id",
    "auth_time": 1533646833,
    "exp": 1533650433,
}