Register in Random Beacon sortition pool

[pdyraga]

Depends on #3005

The client should check if it is eligible to join the sortition pool and if so, join it.

Reference: https://github.com/keep-network/keep-core/blob/main/solidity/random-beacon/contracts/libraries/BeaconAuthorization.sol

Regarding chain-facing implementation, I suggest taking different approach for v2: We should no longer have a single ethereum.go. It’s too big and too complicated. Instead, we should have an Ethereum layer per module. In this case, pkg/sortition/sortition.go and pkg/sortition/chain.go and pkg/sortition/ethereum.go.