Add Query to get all Staker Addresses

[silvermind]

Add a Query Endpoint to get all staker addresses. So one can use these together with calls to all_staked_balances to get more insight in the Alliance Hub. https://terrasco.pe/mainnet/address/terra1jwyzzsaag4t0evnuukc35ysyrx9arzdde2kg9cld28alhjurtthq0prs2s

Atm to to get the list of all staker addresses quering the state is required.