Feature Request: Dynamic backend

[leomoreno310]

I want the choice to specify runtime attribute backend per task. Right now we are able to set backends either by hardcoding or setting defaults in workflow options. I want to be able to pass in a variable to dynamically change the backend I am using.

An example of a pattern I can see working is:

task test1 {
    input {
        String? backend_ = "some_backend"
    }
    runtime {
        backend: backend_
    }
}

Is there a reason why we can't set this now? This dynamic runtime attributes exist now for memory, cpu, docker, etc. but not for backend.