Adds additional user attributes to the user pool schema.
See https://www.paws-r-sdk.com/docs/cognitoidentityprovider_add_custom_attributes/ for full documentation.
cognitoidentityprovider_add_custom_attributes(UserPoolId, CustomAttributes)
[required] The user pool ID for the user pool where you want to add custom attributes.
[required] An array of custom attributes, such as Mutable and Name.