KAFKA-21140: Add randomized testbed for streams group task assignors

[gabriellefu]

• TaskAssignorTestbed — generates a random topology and group from a seed
  (two profiles: SMALL for logical corners, LARGE for production shape), runs the
  assignor until the assignment is a fixed point, then applies random rebalance
  events (process add/drop/restart/retag, member add/drop, standby count change)
  and converges again. A failure prints the seed and the full history; replay a
  single scenario with STREAMS_ASSIGNOR_FUZZ_SEED=<seed> or shift the whole run
  with STREAMS_ASSIGNOR_FUZZ_BASE_SEED=<seed> (system properties work from an IDE).
• AssignmentInvariants — properties every assignor must satisfy, asserted on
  every assignment: output covers exactly the group members, every task is active
  on exactly one member, standbys only on stateful tasks, at most
  numStandbyReplicas standbys per task, no process holds two copies of a task.
• AssignmentMetrics — grades an assignment without asserting: active/total
  spread per member, load spread per process, process/member stickiness, task
  moves, rack diversity per tag. The summary over a run is printed so two versions
  of an assignor can be compared.
• StickyTaskAssignorFuzzTest — runs StickyTaskAssignor through the testbed
  and adds the checks its algorithm guarantees: no member above
  ceil(activeTasks / members), every stateful task gets exactly
  min(numStandbyReplicas, processes - 1) standbys, and rack-aware tags do not
  change the active assignment.