[FEATURE REQUEST] API for retrieving token amount (e.g. stZil)

[b04902036]

Is your feature request related to a problem? Please describe.
After reading the API doc here, seems that there is no API for retrieving total amount of tokens (except zil) of a given address

Describe the solution you'd like
Not sure currently how wallets are doing it, I'm wondering if it is possible to add this into the GetBalance API

Describe alternatives you've considered
Number of tokens can be retrieved by accumulate them along with all transaction, but it will be quiet slow

Additional context
I do notice there is this doc about getting staking amount of a delegator from a ssnlist node, however not sure if I type in the ssnlist node address wrong, always, get empty response

An example of my request

[zil15lr86jwg937urdeayvtypvhy6pnp6d7p8n5z09](https://devex.zilliqa.com/address/zil15lr86jwg937urdeayvtypvhy6pnp6d7p8n5z09)
curl -d '{
     "id": "333",
     "jsonrpc": "2.0",
     "method": "GetSmartContractSubState",
     "params": ["zil15lr86jwg937urdeayvtypvhy6pnp6d7p8n5z09","deposit_amt_deleg",["zil1skteskq34wmvej5lvs5ppvgw58f417mtn54w4j"]]
 }' -H "Content-Type: application/json" -X POST "https://api.testnet.zilliqa.com/"

Thanks a lot for help!

[rrw-zilliqa]

For a ZRC-2 token, you can get the contract state (or substate) and look in the map. For an ERC-20 token, use balanceOf() (or I guess, if you can work out where the storage slot is, you can do it using eth_getStorageAt() )