Ensure coders are updatable partially in a cluster (aka add versions to coders)

Follow up ticket for the following discussion: https://lists.apache.org/thread.html/f3b2daa740075cc39dc04f08d1eaacfcc2d550cecc04e27be024cf52@%3Cdev.beam.apache.org%3E

Long story short: seems being able to update a live stream is a constraint beam must address. This implies no change in coder serialization. To do it the only way is to put a marker (like a version) otherwise the live update will be broken if part of a cluster is updated only.

This task is about ensuring we support it.