Add AssignedBuildId to workflow describe

[ShahabT]

What was changed

workflow describe now shows WF's assigned build ID as follows:

$temporal --env shahab-test4 workflow describe --workflow-id b72b1a82-7a83-4cc9-9bde-6ad8d411a47c      
Execution Info:
  WorkflowId            b72b1a82-7a83-4cc9-9bde-6ad8d411a47c
  RunId                 c81a42c2-efc1-4c41-9088-a0123c0d1034
  Type                  MyWorkflow
  Namespace             shahab-test4.temporal-dev
  TaskQueue             my-tq
  AssignedBuildId       BUILD_ID_HERE
  StartTime             4 hours ago
  ExecutionTime         4 hours ago
  SearchAttributes      map[BuildIds:metadata:{key:"encoding" value:"json/plain"} metadata:{key:"type" value:"KeywordList"} data:"[\"assigned:C\",\"versioned:C\"]"]
  StateTransitionCount  2
  HistoryLength         2
  HistorySize           263

Why?

This is an important attribute for WFs running on versioned workers.

Checklist

1. Closes

2. How was this tested:

Unit test added.

3. Any docs updates needed?

No.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[ShahabT]

Waiting for #556 to merge first.