Support getBalance API #169
Description
Hello, Paypay SDK team.
I think that SDK only supports check balance api(https://github.com/paypay/paypayopa-sdk-java/blob/master/docs/WalletApi.md#checkWalletBalance)
Any plans to support getBalance api(https://www.paypay.ne.jp/opa/doc/v1.0/get_balance.html#operation/getBalance)?
Please review whether support is possible. If it is late, I can work quickly and provide PR.