TreeSnapshot
Aggregate of all the snapshots of a tree of workflows.
Can be serialized with toByteString and deserialized with parse.
For tests, you can get a TreeSnapshot from a RenderingAndSnapshot or by creating one for your root workflow only by calling forRootOnly.
Parameters
childTreeSnapshots
A function that will lazily parse the child snapshots for this tree when invoked. It will be cached in the childTreeSnapshots property.