Refine PlacementRule interface and add a app-name queue mapping rule as an example

After YARN-3635/YARN-6689, PlacementRule becomes a common interface which can be used by scheduler and can be dynamically updated by scheduler according to configs. There're some other works.

• There's no way to initialize PlacementRule.
• No example of PlacementRule except the user-group mapping one.

This JIRA is targeted to refine PlacementRule interfaces and add another PlacementRule example.