Make collections config reference based

This is from https://stackoverflow.com/questions/68635379/hyperledger-fabric-adding-new-members-to-channel-with-private-data-can-i-do/68643959#68643959 with the description being:
"""
As of now, every collection configuration is an explicit policy, so you need to perform an operation for each collection.

However, the design / architecture doesn't restrict the policy to be an explicit one, and is flexible enough to accommodate a dynamic policy that is referenced and you can then update just the reference once instead of updating each collection.

Sadly, this isn't implemented yet. If you open a JIRA / Github issue / send an email in the mailing list then maybe it will be implemented at some point in the future.

It shouldn't be too hard to implement. All we need is to:

Stick a policy in the channel config tree.
Extend the CollectionPolicyConfig with another option of a policy reference and use the aforementioned reference.

"""