Add JobParameters that collates job parameters and id into one

[adam-fowler]

Instead of having

struct EmailJob {
    let to: String
    let subject: String
    let body: String
}
extension JobIdentifier<EmailJob> {
    static let emailJob: Self { "EmailJob" }
}

This allows you to collate everything into the one type

struct EmailJob {
    static jobID: String = "EmailJob"
    let to: String
    let subject: String
    let body: String
}