New APIs needed for tracking lead controller and table assignment

[jackjlli]

After Helix controller and Pinot controller are logically separated (#3957), the periodic tasks and realtime segment completion for all the tables will be spread into 24 partitions. It's necessary to have some APIs to track the lead controller and table assignment.

E.g.:

1. given a table called testTable, I wanna know which Pinot controller works on this table.
2. show me all the table assignments within this cluster.

[xiangfu0]

Closing — this has been implemented in PR #4545. REST APIs added in PinotLeadControllerRestletResource for tracking lead controller and table assignment.