Add delegation rate to the reward response

[cspramit]

We should return the delegation rate for the given era and validator in the reward information responses.

[jacek-casper]

https://github.com/casper-network/casper-node/pull/4770 https://github.com/casper-network/casper-sidecar/pull/328 https://github.com/casper-ecosystem/casper-client-rs/pull/179