feat: Change api attributes from private to protected

[ernieyang09]

it's useful for below scenario.

class ExtendTonClient extends TonClient {
    construct(params) {
        super(params)
        this.api = ....
    }
}

[dvlkv]

Hi! can you please resolve conflicts?

[ernieyang09]

Thanks, since it removes #sign and use private, only change private keyword to protected.