remove direct grpc type StatusCode from public api

[rekby]

StatusCode

    pub fn operation_status(&self) -> YdbResult<StatusCode> {
        StatusCode::from_i32(self.operation_status).ok_or_else(|| {
            YdbError::InternalError(format!("unknown status code: {}", self.operation_status))
        })
    }

[kon3gor]

Hi @rekby I'd like to work on this issue, don't you mind?

[rekby]

@kon3gor , it will great, thanks!