Add `parent_wf_run_id` to `rpc SearchWfRun`

[coltmcnealy-lh]

We should add a field:

optional WfRunId parent_wf_run_id = ?;

to the message SearchWfRunRequest. This will allow users to restrict the results from an rpc SearchWfRun to only those that are children of a specific WfRun.

As for implementation, here are some notes:

• We currently don't have a query planner, because we do not have Counted Tags.
• It is most likely that the most efficient way to do this would be:
  • Object Id Prefix Scan based on parent wfrun id
  • Execute other filters, such as status/variables/wfSpecName, while iterating over children.

In the long term, we need to optimize this. How?

• Implement COUNTED tags, which allow us to know:
  • How many children does a WfRun have?
  • How many WfRun's of a certain WfSpec have a given status?
  • How many WfRun's have a variable matching a certain value?
• Choose the dimension with the fewest elements, and iterate over that dimension first.

[coltmcnealy-lh]

@diablouma had requested this for the control plane. We won't add it quite yet...but the short-term implementation is no more than a half day of work.

Maybe Nelson or Mateo could take this when time permits? Alternatively, it might be a good starter issue for our new hire.