Support for ProxyCommand in sshtunnel.py

[MaxMotovilov]

A very simple change to use ProxyCommand facility in Paramiko in order to support host jumping mehod described in https://en.wikibooks.org/wiki/OpenSSH/Cookbook/Proxies_and_Jump_Hosts#Passing_Through_a_Gateway_Using_stdio_Forwarding_(Netcat_Mode)

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[mysql-oca-bot]

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[MaxMotovilov]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

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[mike-lischke]

Thanks for the contribution, however we stopped using Paramiko in the 8.x series of MySQL Workbench. Instead we switched to libssh.