Add endpoint for validator staking

[bedeho]

Background

#38

Proposal

Add new endpoint validator_staking which returns a JSON discretionary with mappings

• recorded_at_block: The Number for the black at which this data was recorded.
• recorded_at_time: Clock time of recording (must not be computed from blocks, but actual block header clock)
• number_of_validators:the number of validators in the validator set
• number_of_waiting_validators: number of people trying to become a validator
• stake_required_to_become_validator: current amount of $JPY needed to win a spot
• validator_reward: the current amount of reward paid out to a validator. Here their may be nuances, perhaps it is per $JOY, or perhaps it depends on other factors. If needed, just rename to validator_reward_estimate to accommodate this if it gets very tricky.
• total_staking: all tokens, across validators and nominators, involved in staking for validation.
• apr: the APR for a $JOY token involved in staking. Again, if this needs to be an estimate, update name

[bedeho]

It may be useful to cross reference how this data is fetched, or not, in the upcoming validator dashboard in Pioneer.