[Staking] Return APR for a validator somehow

[hewigovens]

@wpank shared to us how to calculate APR based on chain data, would be great if sidecar implements this

First query for the activeEra:

SIDECAR_URL/pallets/staking/storage/activeEra

Then query for how much is currently staked:

SIDECAR_URL//pallets/staking/storage/erasTotalStake?key1=<ACTIVE_ERA>

Then query for how much total issuance there is:

SIDECAR_URL/pallets/balances/storage/totalIssuance

Then use stakedRate / TotalIssuance as an input for the above function

[dvdplm]

I like this idea.

[emostov]

Given how specific this is in to the relay chain we have decided to hold off on adding this feature for now.

Once #488 is it done, it may make sense to revisit this issue and see if either the maintainers here or a third party might want to a make pluggable version of this functionality.