Expose workflow annotations and labels to pods

[discordianfish]

FEATURE REQUEST

Argo should support exposing the workflow annotations and labels to pods.

Rational

All Kubernetes objects have metadata, that makes it a good place to pass in data in a object agnostic way.

Specifically I'm working on a webhook handler that can apply/create arbitrary objects. I want to use that for submitting workflows. To make passing in arguments agnostic to the type of resource I apply, I want to use the metadata field.

Implementation

I believe the best option is to expose these as new global templating variable workflow.metadata.annotations and workflow.metadata.annotations.