Add current stNEAR balance for each mpDAO round voter entry

[akaia-shadowfox]

Requirements

Need changes to the /api/v1/mpdao/voter-info endpoint response:

• Add current stNEAR balance on Metapool liquid staking contract as staking_token_balance string ( stringified U128 amount ) property. No need to keep this value updated since the voting round is already finished
• Add staking_token_id property that exposes the Metapool liquid staking contract address

Notes

staking_token_id by environment:

test	staging	production
meta-v2.pool.testnet	meta-pool.near	meta-pool.near