Define fromParam and fromVar

[mikkelhegn]

Quoting @hongchaodeng from #146

In fact we haven't defined fromParam function yet.
The fromParam is used only as explicit fields. But I could see use cases like this would need to take fromParam.
Maybe we'd better change fromParam fields into built-in function, the same as fromVar?

[technosophos]

fromParam only appears in components, which are not rendered as templates under any circumstances. It is defined here: https://github.com/oam-dev/spec/blob/master/3.component_model.md

The [fromVar(NAME)] function is only interpreted in the AppConfig. And I believe there is an issue to remove variables, as they seem to be redundant with the tooling used to deploy manifests.

So my preference would be to not create a fromParam function and leave it as it is. That said, I wouldn't mind renaming it to valueFrom, which is similar to the way Kubernetes secrets and config maps are mounted to pods: https://kubernetes.io/docs/tasks/configure-pod-container/configure-pod-configmap/#use-configmap-defined-environment-variables-in-pod-commands