[SERVICES-2586] Add staking min unbound epochs field to staking proxy model

[mad2sm0key]

Reasoning

• Currently, the front end needs to perform an additional query for fetching minUnbondEpochs for a staking farm corresponding to a staking proxy

Proposed Changes

• add stakingMinUnboundEpochs field to StakingProxy model
• add resolver method for the newly added field

How to test

stakingProxies {
    address
    stakingFarmAddress
    stakingMinUnboundEpochs
    farmToken {
      ticker
    }
    lpFarmToken {
      ticker
    }
  }