Unable to specify List as return type for Mapping

[NOtherDev]

Specifying List type as a value in Mapping is supported by CloudFormation:

Mapping: {
    Certificates: {
        stage: {
            ARNs: [
                {CertificateArn: "ASDF"},
                {CertificateArn: "GHJK"},
            ]
        }
    }
}

This is a valid construct, but cloudform rejects it expecting string | number | string[] as the 2nd-level value.