Add view method to staking precompile to access stake map

[gztensor]

We need a view method in staking precompile that allows to get stake by hotkey, coldkey, and netuid (only accept root ID for netuid for now, but it will need to change later).

In Solidity/EVM Substrate keys are represented as bytes32 public keys.

Comment: If a H160 address stakes, there's still a 32 byte public key that represents it in stake map in subtensor pallet, so there is no need to allow accessing stake map by H160 address in addition to 32 byte substrate public key.