task.set_*stream should have more keyword arguments

[cicdw]

Task.set_upstream and Task.set_downstream were originally envisioned as methods for building up a Flow which does not pass data. Task.set_dependencies offers a richer API for specifying data passage between tasks.

We should elevate all these methods to expose the full set of Prefect's features:

• allow for the creation of keyed edges
• allow for the creation of mapped edges

We might need to think about whether set_upstream and set_downstream should have the same exact options - calls where the upstream task specifies a key on the downstream task like t1.set_downstream(t2, key="y") feel unnatural.