[Staking] Delegated staking

[raulk]

Users have expressed the desire to have IPC support delegated staking patterns, especially once for token-bound collateral is introduced (#1024). Given the right implementation of #1079, the core flows supporting delegated staking could live entirely in user/app-land, therefore requiring nothing else from IPC Core development.

Base flow:

1. Validator reward contract on the L1 acts as a management entrypoint for validators, collateral, and rewards. It transfers stake to the corresponding gateway actor.
2. Children subnets embed validator activity reports within every bottom-up checkpoint. These reports are rolled-up all the way up to the parent.
3. With every L2=>L1 checkpoint, the corresponding subnet actor relays validator activity reports to the validator reward for reward disbursement.

Delegated staking flow:

1. A delegating party turns up to the validator reward contract, deposits tokens as collateral, and delegates those tokens to specific validators.
2. On reward disbursement, the validator reward contract handles the apportionment of rewards between delegates and delegators, according to some contract-specific logic.