[CT-2041] Accept optional 'version' keyword argument to ref

[MichelleArk]

Add the optional version keyword argument to the ref method/macro.

ref('model', version=2)

Expected lookup behaviour:

• If version keyword argument is provided:
  • provide version to target node lookup in manifest.
• Otherwise:
  • Try to lookup non-versioned target node, returning it if found.
  • Lookup all versions of the target node, and return the node with latest:true.

[emmyoop]

It's unclear how this ref will work. Are there multiple unique ids or do the various versions of a single model live in a list?

[MichelleArk]

Closing in favor of #7263