implement execute_cw20 and query_cw20 function for Cw4626

[pacmanifold]

We should implement helper functions that handles the all the valid Cw20 messages for a Cw4626ExecuteMsg and Cw4626QueryMsg, so that these can easily be called from contract.rs in contracts trying to implement a default Cw4626 implementation. Should look something like this

pub fn cw20_execute<T>(deps: DepsMut, env: &Env, info: &MessageInfo, msg: Cw4626ExecuteMsg<T>) -> Result<Response, CwTokenError>

pub fn cw20_query<T>(deps: Deps, env: &Env, msg: Cw4626ExecuteMsg<T>) -> StdResult<Binary>