feat!: support credentials vending write

[jackye1995]

This PR completes support for credentials vending write

We also make a backwards incompatible change because namespace is not serializable, and we need to reconstruct namespace to do credentials refresh, we instead pass in namespace_impl and namespace_properties so that they are passed around to initialize namespace client in individual workers.

cc @bryanck @jtuglu1

[jtuglu1]

Cool. Have not taken a deep look yet but is the pattern that every worker will manage their own credential lifetime (e.g. have credential refreshes linear in the # of fragment batches)? E.g. no way for head node to manage refresh once and broadcast it to the workers to avoid something like thundering herd of refreshes?