Add the ability to turn on data compression for all accounts

[diwu1989]

SoftEther is supposed to support data compression. Can we make it easy with 1 flag to turn on data compression for all the generated accounts?

https://www.softether.org/4-docs/1-manual/6.Command_Line_Management_Utility_Manual/6.5_VPN_Client_Management_Command_Reference#6.5.32.22AccountCompressEnable.22:_Enable_Data_Compression_when_Communicating_by_VPN_Connection_Setting

[siomiz]

Client management is not handled by this image. (PRs are welcome)

If you know an equivalent vpncmd command for Server and/or Hub (to force default or something), use VPNCMD_SERVER and/or VPNCMD_HUB.