add support for tunneling SFTP connections through proxy

[rsioss]

Customers frequently need to connect to their trading partner's SFTP server via allow-listed static IP addresses. One workaround that we frequently suggest to customers is to connect through an external proxy server that has a static address (or set of addresses).

In order to support this in bootstrap, we need to support proxy-related options in the SFTP configuration schema. At a minimum, this would be the proxy host and port, but other options may be useful as well. Some example code for connecting via a Socks v5 proxy can be found here:

theophilusx/ssh2-sftp-client#39 (comment)