Extend Opera-specific API calls

[uprendis]

• Add epoch number into header/block API output
• Add abft_getValidators API for retrieving validator weights and public keys
• Add ftm_getRules API call for retrieving network rules (e.g. throughput limits or upgrades activation status)
• Add ftm_getEpochBlock API call for retrieving block height in a beginning of an epoch

[hadv]

LGTM, it's a bit confuse when using rpc.BlockNumber for epoch but we already used it as it is before.